Run PR ld/17618 test only with 64-bit ELF linker

Run PR ld/17618 test only with 64-bit ELF linker

PR ld/17618 test requires 64-bit linker to run.  Set LD_CLASS to "64bit"
for 64-bit ELF linker and run PR ld/17618 test only if $LD_CLASS is
"64bit".  More checks can be added to support 64-bit linkers in non-ELF
format.

	* testsuite/config/default.exp (LD_CLASS): New.  Set to "64bit"
	for 64-bit ELF linker.
	* testsuite/ld-x86-64/pr17618.d (#notarget): Removed.
	* testsuite/ld-x86-64/x86-64.exp: Run pr17618 only for 64-bit
	linker.