Remove use of deprecated core functions (in NetBSD/ARM)

This is in preparation for deleting deprecated_add_core_fns and
related code.

As a side-effect, this makes it possible to read NetBSD/ARM
core files on non-NetBSD/ARM platforms, subject to PR corefiles/25638.

I have removed this comment:
-  /* This is ok: we're running native...  */
Since we are using the gdbarch from the regcache, we should be
guaranteed to be calling the right function here, so it shouldn't
matter whether we are running native.

Tested by reading a NetBSD/ARM core file on Linux/x86-64 and NetBSD/ARM;
the "info registers" output matches the one from the system GDB.

gdb/ChangeLog:

2020-03-12  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in (HFILES_NO_SRCDIR): Add new arm-nbsd-tdep.h file.
	* arm-nbsd-nat.c (arm_supply_gregset): Moved to arm-nbsd-tdep and
	renamed to arm_nbsd_supply_gregset.
	(fetch_register): Update to call arm_nbsd_supply_gregset.
	(fetch_regs): Remove in favor of fetch_register with a -1 regno.
	(arm_netbsd_nat_target::fetch_registers): Update.
	(fetch_elfcore_registers): Removed.
	(_initialize_arm_netbsd_nat): Removed call to deprecated_add_core_fns.
	* arm-nbsd-tdep.c (struct arm_nbsd_reg): New struct.
	(arm_nbsd_supply_gregset): Moved from arm-nbsd-nat.c and updated to
	not require NetBSD system headers.
	(arm_nbsd_regset): New struct.
	(arm_nbsd_iterate_over_regset_sections): New function.
	(arm_netbsd_init_abi_common): Updated to call
	set_gdbarch_iterate_over_regset_sections.
	* arm-nbsd-tdep.h: New file.

gdb/arm-nbsd-tdep.c

@@ -21,7 +21,9 @@
 #include "osabi.h"
 
 #include "arch/arm.h"
+#include "arm-nbsd-tdep.h"
 #include "arm-tdep.h"
+#include "regset.h"
 #include "solib-svr4.h"
 
 /* Description of the longjmp buffer.  */
@@ -35,6 +37,75 @@ static const gdb_byte arm_nbsd_arm_be_breakpoint[] = {0xe6, 0x00, 0x00, 0x11};
 static const gdb_byte arm_nbsd_thumb_le_breakpoint[] = {0xfe, 0xde};
 static const gdb_byte arm_nbsd_thumb_be_breakpoint[] = {0xde, 0xfe};
 
+/* This matches struct reg from NetBSD's sys/arch/arm/include/reg.h:
+   https://github.com/NetBSD/src/blob/7c13e6e6773bb171f4ed3ed53013e9d24b3c1eac/sys/arch/arm/include/reg.h#L39
+ */
+struct arm_nbsd_reg
+{
+  uint32_t reg[13];
+  uint32_t sp;
+  uint32_t lr;
+  uint32_t pc;
+  uint32_t cpsr;
+};
+
+void
+arm_nbsd_supply_gregset (const struct regset *regset, struct regcache *regcache,
+			 int regnum, const void *gregs, size_t len)
+{
+  const arm_nbsd_reg *gregset = static_cast<const arm_nbsd_reg *>(gregs);
+  gdb_assert (len >= sizeof (arm_nbsd_reg));
+
+  /* Integer registers.  */
+  for (int i = ARM_A1_REGNUM; i < ARM_SP_REGNUM; i++)
+    if (regnum == -1 || regnum == i)
+      regcache->raw_supply (i, (char *) &gregset->reg[i]);
+
+  if (regnum == -1 || regnum == ARM_SP_REGNUM)
+    regcache->raw_supply (ARM_SP_REGNUM, (char *) &gregset->sp);
+
+  if (regnum == -1 || regnum == ARM_LR_REGNUM)
+    regcache->raw_supply (ARM_LR_REGNUM, (char *) &gregset->lr);
+
+  if (regnum == -1 || regnum == ARM_PC_REGNUM)
+    {
+      CORE_ADDR r_pc = gdbarch_addr_bits_remove (regcache->arch (), gregset->pc);
+      regcache->raw_supply (ARM_PC_REGNUM, (char *) &r_pc);
+    }
+
+  if (regnum == -1 || regnum == ARM_PS_REGNUM)
+    {
+      if (arm_apcs_32)
+	regcache->raw_supply (ARM_PS_REGNUM, (char *) &gregset->cpsr);
+      else
+	regcache->raw_supply (ARM_PS_REGNUM, (char *) &gregset->pc);
+    }
+}
+
+static const struct regset arm_nbsd_regset = {
+  nullptr,
+  arm_nbsd_supply_gregset,
+  /* We don't need a collect function because we only use this reading registers
+     (via iterate_over_regset_sections and fetch_regs/fetch_register).  */
+  nullptr,
+  0
+};
+
+static void
+arm_nbsd_iterate_over_regset_sections (struct gdbarch *gdbarch,
+				       iterate_over_regset_sections_cb *cb,
+				       void *cb_data,
+				       const struct regcache *regcache)
+{
+  cb (".reg", sizeof (arm_nbsd_reg), sizeof (arm_nbsd_reg), &arm_nbsd_regset,
+      NULL, cb_data);
+  /* cbiesinger/2020-02-12 -- as far as I can tell, ARM/NetBSD does
+     not write any floating point registers into the core file (tested
+     with NetBSD 9.1_RC1).  When it does, this function will need to read them,
+     and the arm-netbsd gdbarch will need a core_read_description function
+     to return the right description for them.  */
+}
+
 static void
 arm_netbsd_init_abi_common (struct gdbarch_info info,
 			    struct gdbarch *gdbarch)
@@ -66,10 +137,12 @@ arm_netbsd_init_abi_common (struct gdbarch_info info,
   tdep->jb_pc = ARM_NBSD_JB_PC;
   tdep->jb_elt_size = ARM_NBSD_JB_ELEMENT_SIZE;
 
+  set_gdbarch_iterate_over_regset_sections
+    (gdbarch, arm_nbsd_iterate_over_regset_sections);
   /* Single stepping.  */
   set_gdbarch_software_single_step (gdbarch, arm_software_single_step);
 }
-  
+
 static void
 arm_netbsd_elf_init_abi (struct gdbarch_info info,
 			 struct gdbarch *gdbarch)