bfd/riscv: tighten matching rules in riscv_scan

The following GDB behaviour was observed:

  (gdb) x/1i 0x0001014a
     0x1014a <main+8>:	jal	0x10132 <foo>
  (gdb) show architecture
  The target architecture is set automatically (currently riscv:rv32)
  (gdb) set architecture riscv:rv32
  The target architecture is assumed to be riscv:rv32
  (gdb) x/1i 0x0001014a
     0x1014a <main+8>:	0x37e5
  (gdb)

Notice that initially we can disassemble the instruction (it's a
compressed jal instruction), but after setting the architecture we can
no longer disassemble the instruction.

This is particularly puzzling as GDB initially thought the
architecture was 'riscv:rv32', but when we force the architecture to
be that, the disassembly stops working.

This issue was introduced with this commit:

  commit c35d018b1a5ec604e49a807402c4205530b25ca8
  Date:   Mon Jan 27 15:19:30 2020 -0800

      RISC-V: Fix gdbserver problem with handling arch strings.

In this commit we try to make riscv_scan handle cases where we see
architecture strings like 'riscv:rv32imc' (for example).  Normally
this wouldn't match as bfd_default_scan requires an exact match, so we
extended riscv_scan to ignore trailing characters.

Unfortunately the default riscv arch is called 'riscv', is 64-bit,
and has its mach type set to 0, which I think is intended to pair with
code is riscv-dis.c:riscv_disassemble_insn that tries to guess if we
are 32 or 64 bit.

What happens then is that 'riscv:rv32' is first tested against 'riscv'
using bfd_default_scan, this doesn't match, we then compare this to
'riscv', but allowing trailing characters to be ignored, this matches,
and our 'riscv:rv32' matches against the default (64-bit)
architecture.

The solution I propose is to prevent the default architecture from
taking part in this "ignore trailing characters" extra match case,
only the more specific 'riscv:rv32' and 'riscv:rv64' get this extra
matching.

bfd/ChangeLog:

	* cpu-riscv.c (riscv_scan): Don't allow shorter matches using the
	default architecture.

bfd/ChangeLog

@@ -1,3 +1,8 @@
+2020-06-24  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* cpu-riscv.c (riscv_scan): Don't allow shorter matches using the
+	default architecture.
+
 2020-06-24  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR ld/26083

bfd/cpu-riscv.c

@@ -47,10 +47,20 @@ riscv_scan (const struct bfd_arch_info *info, const char *string)
   if (bfd_default_scan (info, string))
     return TRUE;
 
-  /* The string might have extra characters for supported subsets.  So allow
-     a match that ignores trailing characters in string.  */
-  if (strncasecmp (string, info->printable_name,
-		   strlen (info->printable_name)) == 0)
+  /* The incoming STRING might take the form of riscv:rvXXzzz, where XX is
+     32 or 64, and zzz are one or more extension characters.  As we
+     currently only have 3 architectures defined, 'riscv', 'riscv:rv32',
+     and 'riscv:rv64', we would like to ignore the zzz for the purpose of
+     matching here.
+
+     However, we don't want the default 'riscv' to match over a more
+     specific 'riscv:rv32' or 'riscv:rv64', so in the case of the default
+     architecture (with the shorter 'riscv' name) we don't allow any
+     special matching, but for the 'riscv:rvXX' cases, we allow a match
+     with any additional trailing characters being ignored.  */
+  if (!info->the_default
+      && strncasecmp (string, info->printable_name,
+                      strlen (info->printable_name)) == 0)
     return TRUE;
 
   return FALSE;