In powerpc ncg, add a speed hack for sti 8.

In powerpc ncg, add a speed hack for sti 8.

ncg is too slow with this many registers.  A stack pattern "with GPR
GPR GPR" or "with REG REG REG" takes too long to pick registers,
causing ncg 8 to take about 2 seconds on each sti 8.  I introduce
REG_PAIR and there are only 4 such pairs.

For programs that use sti 8 (including C programs that copy 8-byte
structs), this speed hack improves the ncg run from several seconds to
almost instantaneous.

Also add a few COMMENT(...) lines in stacking rules.