[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*".