gdbserver: Add LoongArch/Linux support

Implement LoongArch/Linux support, including XML target description
handling based on features determined, GPR regset support, and software
breakpoint handling.

In the Linux kernel code of LoongArch, ptrace implements PTRACE_POKEUSR
and PTRACE_PEEKUSR in the arch_ptrace function, so srv_linux_usrregs is
set to yes.

With this patch on LoongArch:

  $ make check-gdb TESTS="gdb.server/server-connect.exp"
  [...]
  # of expected passes		18
  [...]

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

gdb/arch/loongarch.c

@@ -25,7 +25,13 @@
 #include "../features/loongarch/base32.c"
 #include "../features/loongarch/base64.c"
 
-static target_desc_up
+#ifndef GDBSERVER
+#define STATIC_IN_GDB static
+#else
+#define STATIC_IN_GDB
+#endif
+
+STATIC_IN_GDB target_desc_up
 loongarch_create_target_description (const struct loongarch_gdbarch_features features)
 {
   /* Now we should create a new target description.  */
@@ -51,6 +57,8 @@ loongarch_create_target_description (const struct loongarch_gdbarch_features fea
   return tdesc;
 }
 
+#ifndef GDBSERVER
+
 /* Wrapper used by std::unordered_map to generate hash for feature set.  */
 struct loongarch_gdbarch_features_hasher
 {
@@ -86,3 +94,5 @@ loongarch_lookup_target_description (const struct loongarch_gdbarch_features fea
   loongarch_tdesc_cache.emplace (features, std::move (tdesc));
   return ptr;
 }
+
+#endif /* !GDBSERVER */

gdb/arch/loongarch.h

@@ -60,6 +60,17 @@ struct loongarch_gdbarch_features
   }
 };
 
+#ifdef GDBSERVER
+
+/* Create and return a target description that is compatible with FEATURES.
+   This is only used directly from the gdbserver where the created target
+   description is modified after it is return.  */
+
+target_desc_up loongarch_create_target_description
+	(const struct loongarch_gdbarch_features features);
+
+#else
+
 /* Lookup an already existing target description matching FEATURES, or
    create a new target description if this is the first time we have seen
    FEATURES.  For the same FEATURES the same target_desc is always
@@ -70,4 +81,6 @@ struct loongarch_gdbarch_features
 const target_desc *loongarch_lookup_target_description
 	(const struct loongarch_gdbarch_features features);
 
+#endif /* GDBSERVER */
+
 #endif /* ARCH_LOONGARCH_H  */