mk for only supported archs

mk for only supported archs
http://fqa.9front.org/fqa1.html#1.2 states the supported archs.
However, clean and nuke also remove build files for 0 (spim) and q
(power).  'mk all' using those archs fails; 'mk kernels' also tries to
build all the kernels, even those which are not supported.  For
example, I tried to build the power arch (qc, qa, ql) and without
surprise it failed (when building dtracy): ...
mk dtracy
qc -FTVw dtracy.c
yacc -v -d -D1 parse.y
qc -FTVw cgen.c
qc -FTVw act.c
qc -FTVw type.c
 == regfree ==
REGISTER R0 <11> STRUCT DTAct cgen.c:302
 == regfree ==

REGISTER R0 <11> STRUCT DTAct act.c:266

 == regfree ==
qc -FTVw agg.c
cgen.c:299 unknown type in regalloc: STRUCT DTAct
cgen.c:299 bad opcode in gmove INT -> STRUCT DTAct
cgen.c:302 unknown type in regalloc: STRUCT DTAct
cgen.c:302 bad opcode in gmove INT -> STRUCT DTAct
cgen.c:302 error in regfree: 0 [0]
REGISTERmk: qc -FTVw cgen.c  : exit status=rc 387386: qc 387392: error R0
 <11> STRUCT DTAct act.c:269

act.c:250 unknown type in regalloc: STRUCT DTAct
act.c:250 bad opcode in gmove INT -> STRUCT DTAct
act.c:266 unknown type in regalloc: STRUCT DTAct
act.c:266 bad opcode in gmove INT -> STRUCT DTAct
act.c:266 error in regfree: 0 [0]
act.c:269 unknown type in regalloc: STRUCT DTAct
act.c:269 bad opcode in gmove INT -> STRUCT DTAct
act.c:269 error in regfree: 0 [0]
act.c:274 unknown type in regalloc: STRUCT DTAct
act.c:274 bad opcode in gmove INT -> STRUCT DTAct
act.c:274 error in regfree: 0 [0]
too many errors
mk: for(i in cc ...  : exit status=rc 382748: rc 387379: mk 387381: error
mk: date for (i ...  : exit status=rc 373781: rc 382226: mk 382227: error
cpu%

The patch below skips over non-supported architectures.  Is that
something we want?  This way, 'mk kernels' should work without a
problem (tested on amd64).  Then if someone works on getting those
architectures supported again in the future, they can be added back
in.