suppress possibly used uninitialized warning on sgimips

from martin@
Change efs from hashlist to vcache.
Update for xorg
Drop extra ().
Use cmpwi.
Panic after openfirm, it is not supposed to return.
Use cmpwi.
Use cmpw.
Add missing Kyuafile.
In the usage message, mention that the -m arg may be an alias.
Also adjust a comment around alias processing.
I need 2244k for src/distrib/macppc/floppies/ramdisk, rather than
2240k.  My build is normal, with a few uncommitted changes that really
shouldn't change the install situation.  Because 2240k isn't near a
magic boundary, just increase it a hair.
Get macppc to build again by adjusting sizes.

In addition to the ramdisk size, it is necessary to have matching MD
sizes.  Increase the MD sizes, and leave a comment near the ramdisk
size to ease future changes.
cast to LHS type and minor KNF.
Add a list-arch operation to build.sh.

This lists all known MACHINE/MACHINE_ARCH values, optionally
filtered by glob patterns given in the "-m" and "-a" options.
For example: build.sh -a '*arm*' list-arch
MACHINE_ARCH from the environment may be overridden by getarch (based
on the MACHINE), but MACHINE_ARCH from the -a option should not be
overridden.  The previous commit accidentally removed this behaviour.
remove obsolete comment
scanning fixes
convert to unsigned multiplication.
Document build.sh list-arch
Add advice about avoiding common pitfalls.

This is taken from the patch in PR 48741, by David H. Gutteridge,
but I made small changes to wording and markup.