GENCGC on SPARC/Linux

GENCGC on SPARC/Linux

  * Tweak make-config.sh to automatically select GENCGC, and to
update the fallback message for those poor bsd-using folks.

  * Copied the gencgc memory map from the SunOS version to use for
Linux (the cheneygc maps were identical, we may as well keep the
parallelism).

  * Also tweaked the linker flags to match.  Not because it was
required to build, but because I don't know why they were changed
for SunOS, and in the absence of such knowledge I'd rather keep
things more or less parallel.