Fix extraneous complaints about missing expected TLS relocation.

With some versions of gas, the call to tls_get_addr uses a GOTPCREL
relocation instead of a GOTPCRELX relocation. We should allow for that
when skip_call_tls_get_addr_ is true. We should also build the test
objects with the in-tree assembler.

This patch also fixes some cascading error messages caused by not
resetting the skip_call_tls_get_addr_ flag after printing the error.

gold/
	PR gold/20216
	* x86_64.cc (Target_x86_64::Relocate::relocate): Add check for
	R_X86_64_GOTPCREL. Reset skip_call_tls_get_addr_ after printing
	error message.
	* testsuite/Makefile.am (pr20216_gd.o): Add -Bgcctestdir/.
	(pr20216_ld.o): Likewise.
	* testsuite/Makefile.in: Regenerate.