PowerPC -many

[Alan Modra]

Since we've been talking about obsoleting cpu support, how about
getting rid of -many in ASM_CPU_SPEC for gcc-8?

It's a horrible hack of mine to work around gcc -mcpu option handling
bugs which I think have been fixed, and to silence complaints from gas
about asm() written for multiple cpus (with presumably run-time
selection of which block of code gets executed depending on cpu).

It used to be just a linux hack, but I see David uses it in aix61.h
and aix71.h too?

-- 
Alan Modra
Australia Development Lab, IBM