add reference to PR for sparc64 BSD_CRTBEGIN in BROKEN_OPTIONS

We will soon remove the BSD_CRTBEGIN option (and will use the new CRT
files always) as part of the GCC 4.2.1 removal. Right now BSD_CRTBEGIN
works everywhere but sparc64; add a reference to the PR in case anyone
stumbles across this and is looking for more information.