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

Following 0becc27, `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).

Author: hubert-reinterpretcast

lld/test/ELF/ppc64-pcrel-long-branch-error.s

@@ -1,3 +1,7 @@
+## The test as-is needs a large heap size.
+## Disabled until we know how to check for that prerequisite.
+# UNSUPPORTED: ppc
+
 # REQUIRES: ppc
 # RUN: echo 'SECTIONS { \
 # RUN:       .text_low 0x2000: { *(.text_low) } \