RISC-V: Re-define mapping symbol $x to the file elf architecture attribute

The mapping symbol "$x" without an ISA string "means using ISA
configuration from ELF attribute."[1].  Currently the code does not
reset the subset_list.  This means that a previous mapping symbol that
overrides the ISA string will continue to be used, rather than the
default string set in the ELF file's .riscv.attributes section.  This
can cause incorrect or failed instruction decodings.

In practice, this causes problems when disassembling code generated by
LLVM, which (unlike gas) does not emit explicit mapping symbols at the
start of each section.

This change stores the default architecture string seen at the beginning
of disassembly in the global parse data struct, and restores that to
subset_list whenever a bare "$x" symbol is seen.

[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#mapping-symbol

Before this patch, the mapping-x.s was dumped as,

00000000 <.text>:
   0:	00000013          	nop
   4:	0001                	.insn	2, 0x0001
   6:	0001                	.insn	2, 0x0001

Which is caused by the definiation of $x was conflict with the psABI.

3 files changed, 32 insertions, 6 deletions

diff --git a/gas/testsuite/gas/riscv/mapping-x.d b/gas/testsuite/gas/riscv/mapping-x.d
new file mode 100644
index 0000000..ccf9ef7
--- /dev/null
+++ b/gas/testsuite/gas/riscv/mapping-x.d
@@ -0,0 +1,13 @@
+#as:
+#source: mapping-x.s
+#objdump: -d
+
+.*:[ 	]+file format .*
+
+
+Disassembly of section .text:
+
+0+000 <.text>:
+[ 	]+[0-9a-f]+:[ 	]+00000013[ 	]+nop
+[ 	]+[0-9a-f]+:[ 	]+0001[ 	]+nop
+[ 	]+[0-9a-f]+:[ 	]+0001[ 	]+nop
diff --git a/gas/testsuite/gas/riscv/mapping-x.s b/gas/testsuite/gas/riscv/mapping-x.s
new file mode 100644
index 0000000..f17cf22
--- /dev/null
+++ b/gas/testsuite/gas/riscv/mapping-x.s
@@ -0,0 +1,6 @@
+.attribute arch, "rv32ic"
+.option arch, -c
+.insn 4, 0x00000013
+$x:
+.insn 2, 0x0001
+.insn 2, 0x0001
diff --git a/opcodes/riscv-dis.c b/opcodes/riscv-dis.c
index 9a63499..c5e9362 100644
--- a/opcodes/riscv-dis.c
+++ b/opcodes/riscv-dis.c
@@ -52,6 +52,9 @@ struct riscv_private_data
   enum riscv_spec_class default_priv_spec;
   /* Used for architecture parser.  */
   riscv_parse_subset_t riscv_rps_dis;
+  /* Default architecture string for the object file.  It will be changed once
+     elf architecture attribute exits.  This is used for mapping symbol $x.  */
+  const char* default_arch;
   /* Used for mapping symbols.  */
   int last_map_symbol;
   bfd_vma last_stop_offset;
@@ -1065,10 +1068,14 @@ riscv_update_map_state (int n,
     return;
 
   name = bfd_asymbol_name(info->symtab[n]);
-  if (strcmp (name, "$x") == 0)
-    *state = MAP_INSN;
-  else if (strcmp (name, "$d") == 0)
+  if (strcmp (name, "$d") == 0)
     *state = MAP_DATA;
+  else if (strcmp (name, "$x") == 0)
+    {
+      *state = MAP_INSN;
+      riscv_release_subset_list (pd->riscv_rps_dis.subset_list);
+      riscv_parse_subset (&pd->riscv_rps_dis, pd->default_arch);
+    }
   else if (strncmp (name, "$xrv", 4) == 0)
     {
       *state = MAP_INSN;
@@ -1380,7 +1387,7 @@ riscv_init_disasm_info (struct disassemble_info *info)
   pd->riscv_rps_dis.xlen = &pd->xlen;
   pd->riscv_rps_dis.isa_spec = &pd->default_isa_spec;
   pd->riscv_rps_dis.check_unknown_prefixed_ext = false;
-  const char *default_arch = "rv64gc";
+  pd->default_arch = "rv64gc";
   if (info->section != NULL)
     {
       bfd *abfd = info->section->owner;
@@ -1398,12 +1405,12 @@ riscv_init_disasm_info (struct disassemble_info *info)
 						      attr[Tag_b].i,
 						      attr[Tag_c].i,
 						      &pd->default_priv_spec);
-	      default_arch = attr[Tag_RISCV_arch].s;
+	      pd->default_arch = attr[Tag_RISCV_arch].s;
 	    }
 	}
     }
   riscv_release_subset_list (pd->riscv_rps_dis.subset_list);
-  riscv_parse_subset (&pd->riscv_rps_dis, default_arch);
+  riscv_parse_subset (&pd->riscv_rps_dis, pd->default_arch);
 
   pd->last_map_symbol = -1;
   pd->last_stop_offset = 0;