gdb/risc-v: fix ISA string detection for disassembly

Commit 3f61a38 introduced a regression where the ISA string was no
longer detected based on the ELF header. The mechanism was changed from
directly referencing `abfd` to using `disassembler_info->section`, which
was not properly initialized for RISC-V.

The previous implementation ignored the object in scope, leading to
issues such as failing to decode RVV instructions when a library was
compiled as `rv64gcv` and the main application as `rv64gc`.

This patch resolves both problems by initializing
`disassembler_info->section` with the object currently in scope,
ensuring correct ISA string detection during disassembly.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/riscv-tdep.c

@@ -4163,6 +4163,20 @@ riscv_gnu_triplet_regexp (struct gdbarch *gdbarch)
   return "riscv(32|64)?";
 }
 
+/* Implement the "print_insn" gdbarch method.  */
+
+static int
+riscv_print_insn (bfd_vma addr, struct disassemble_info *info)
+{
+  /* Initialize the BFD section to enable ISA string detection depending on the
+     object in scope.  */
+  struct obj_section *s = find_pc_section (addr);
+  if (s != nullptr)
+    info->section = s->the_bfd_section;
+
+  return default_print_insn (addr, info);
+}
+
 /* Implementation of `gdbarch_stap_is_single_operand', as defined in
    gdbarch.h.  */
 
@@ -4429,6 +4443,9 @@ riscv_gdbarch_init (struct gdbarch_info info,
 					  disassembler_options_riscv ());
   set_gdbarch_disassembler_options (gdbarch, &riscv_disassembler_options);
 
+  /* Disassembler print_insn.  */
+  set_gdbarch_print_insn (gdbarch, riscv_print_insn);
+
   /* SystemTap Support.  */
   set_gdbarch_stap_is_single_operand (gdbarch, riscv_stap_is_single_operand);
   set_gdbarch_stap_register_indirection_prefixes