[LLD][PowerPC][test] Disable ELF/ppc64-pcrel-long-branch-error.s

Following 0becc27e, `ppc64-pcrel-long-branch-error.s` fails in some
environments with out-of-memory errors associated with buffering the
output in-memory. Since the alternative of writing to an allocated file
is also known to cause problems, we will disable the test
unconditionally (pending a mechanism to disable the test selectively).