Split off some of the platform-independent stuff (such as as and ncg).

--HG--
branch : dtrg-buildsystem
rename : plat/pc86/build.mk => mach/i386/build.mk
rename : plat/pc86/build.mk => mach/i80/build.mk
rename : plat/pc86/build.mk => mach/i86/build.mk
rename : plat/pc86/build.mk => mach/m68020/build.mk
rename : plat/pc86/build.mk => mach/powerpc/build.mk

plat/linuxppc/descr

@@ -27,7 +27,7 @@ var C_INCLUDES=-I{PLATFORMDIR}/include -I{EM}/share/ack/include/ansi
 name be
 	from .m.g
 	to .s
-	program {EM}/lib/ack/{PLATFORM}/ncg
+	program {EM}/lib/ack/{ARCH}/ncg
 	mapflag -gdb GF=-gdb
 	args {GF?} <
 	stdout
@@ -36,7 +36,7 @@ end
 name asopt
 	from .s
 	to .so
-	program {EM}/lib/ack/{PLATFORM}/top
+	program {EM}/lib/ack/{ARCH}/top
 	args
 	optimizer
 	stdin
@@ -45,7 +45,7 @@ end
 name as
 	from .s.so
 	to .o
-	program {EM}/lib/ack/{PLATFORM}/as
+	program {EM}/lib/ack/{ARCH}/as
 	args - -o > <
 	prep cond
 end