gold: don't invoke IA32 syscall in x86_64 assembly testcase

pr17704a_test.s is a x86_64 assembly file, but it invokes IA32 exit
syscall with "int 0x80".  This causes a segfault on kernels with
CONFIG_IA32_EMULATION disabled.

gold/

	* testsuite/pr17704a_test.s (_start): Invoke x86_64 exit syscall
	instead of its IA32 counterpart.