we don't need to specify -mcpu where compiler default is used,
preserve mtune though; do not -mno-altivec on ppc since altivec
is not default in the first place and doing this will just
break random things that explicitly enable altivec for runtime
checked code; do not use -maltivec on either ppc64le or ppc64
since it's implied by their -mcpu's (power8 for ppc64le which
is implicit, and 970 for ppc64 which is explicit)
also remove -mlong-double-64 for musl targets as that's compiler
default and it's not necessary to pass it; also remove -mabi=elfv2
for both ppc64le and ppc64 since it's implied default
29255b0244