jit: fix test-asm failures on i?86

On i686, test_i386_basic_asm_4 has:
  error: inconsistent operand constraints in an 'asm'
and test_i386_basic_asm_5 has:
  /tmp/libgccjit-9FsLie/fake.s:9: Error: bad register name `%rdi'
  /tmp/libgccjit-9FsLie/fake.s:10: Error: bad register name `%rsi'

This is only intended as a smoketest of asm support, so only run
it on x86_64.

gcc/testsuite/ChangeLog:
	* jit.dg/test-asm.c: Remove i?86-*-* from target specifier.
	* jit.dg/test-asm.cc: Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>