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.