gold: Force a PC-relative reference to .LC0

Force a PC-relative reference to .LC0 with:

__asm__ (".dc.a .LC0 - .");

for all targets.

Tested on x86, powerpc64le and aarch64.

	* testsuite/discard_locals_relocatable_test.c: Force a PC-relative
	reference to .LC0.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>