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

Maciej reported a problem found by his RISC-V gdbserver port.
warning: while parsing target description (at line 4): Target description specified unknown architecture "riscv:rv64id"
warning: Could not load XML target description; ignoring

We only have two arches defined, riscv:rv32 and riscv:rv64.  Both bfd and
gdb are creating arch strings that have extension letters added to the base
architecture.  The bfd_default_scan function requires an exact match, so
these strings fail to map to a bfd_arch.  I think we should ignore the
extension letters in a RISC-V specific scan function.

	bfd/
	* cpu-riscv.c (riscv_scan): New.
	(N): Change bfd_default_scan to riscv_scan.

Change-Id: I096476705e1da5cb8934c5005b1eed2a8989f7a7

bfd/ChangeLog

@@ -1,3 +1,8 @@
+2020-01-27  Jim Wilson  <jimw@sifive.com>
+
+	* cpu-riscv.c (riscv_scan): New.
+	(N): Change bfd_default_scan to riscv_scan.
+
 2020-01-27  Andreas Schwab  <schwab@suse.de>
 
 	* Makefile.am (ALL_MACHINES): Remove cpu-plugin.lo.

bfd/cpu-riscv.c

@@ -39,6 +39,23 @@ riscv_compatible (const bfd_arch_info_type *a, const bfd_arch_info_type *b)
   return a;
 }
 
+/* Return TRUE if STRING matches the architecture described by INFO.  */
+
+static bfd_boolean
+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)
+    return TRUE;
+
+  return FALSE;
+}
+
 #define N(BITS, NUMBER, PRINT, DEFAULT, NEXT)			\
   {								\
     BITS,      /* Bits in a word.  */				\
@@ -51,7 +68,7 @@ riscv_compatible (const bfd_arch_info_type *a, const bfd_arch_info_type *b)
     3,								\
     DEFAULT,							\
     riscv_compatible,						\
-    bfd_default_scan,						\
+    riscv_scan,							\
     bfd_arch_default_fill,					\
     NEXT,							\
     0 /* Maximum offset of a reloc from the start of an insn.  */\