ld/x86-64: Add PR gold/26939 tests

GOTPCRELX relocations can be transformed only when addend == -4.  Add
tests for GOTPCRELX relocations with addend != -4.

	PR gold/26939
	* testsuite/ld-x86-64/pr26939-x32.d: New file.
	* testsuite/ld-x86-64/pr26939.d: Likewise.
	* testsuite/ld-x86-64/pr26939.s: Likewise.
	* testsuite/ld-x86-64/x86-64.exp: Run PR gold/26939 tests.