[gdb/testsuite] Require is_amd64_regs_target in gdb.base/disasm-optim.exp

When running test-case gdb.base/disasm-optim.exp with target board unix/-m32,
we get:
...
 Running disasm-optim.exp ...
 gdb compile failed, disasm-optim.c: Assembler messages:
 disasm-optim.c:35: Error: bad register name `%rip)'
 disasm-optim.c:46: Error: bad register name `%rax)'
 disasm-optim.c:57: Error: bad register name `%rip)'

                 === gdb Summary ===

 # of untested testcases         1
...

Fix this by requiring is_amd64_regs_target instead of istarget
"x86_64-*-linux*".

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-12  Tom de Vries  <tdevries@suse.de>

	* gdb.base/disasm-optim.exp: Require is_amd64_regs_target.

2 files changed, 5 insertions, 1 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 0cae318..7c24f66 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2021-01-12  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/disasm-optim.exp: Require is_amd64_regs_target.
+
 2021-01-12  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.fortran/debug-expr.exp: Add new tests.
diff --git a/gdb/testsuite/gdb.base/disasm-optim.exp b/gdb/testsuite/gdb.base/disasm-optim.exp
index 9acbbe8..ebb0d1f 100644
--- a/gdb/testsuite/gdb.base/disasm-optim.exp
+++ b/gdb/testsuite/gdb.base/disasm-optim.exp
@@ -15,7 +15,7 @@
 
 # This test exercises disassemble /s with optimized and inlined code.
 
-if { ![istarget "x86_64-*-linux*"] } {
+if { ![is_amd64_regs_target] } {
     continue
 }