powerpc: Remove cpu-as-y completely

cpu-as-y is there to force assembler building options. But there is no
need for that. GCC is passed the necessary options and it automatically
pass the appropriate option to GAS.

GCC is given -maltivec when relevant, so no need for -Wa,-maltivec
either.

And -Wa,-many is wrong as it will hide innapropriate instructions.
Better to detect them and handle them on a case by case basis.

The setting of -Wa,-many was added by commit 960e3002
("powerpc/Makefile: Fix PPC_BOOK3S_64 ASFLAGS") in order to fix an issue
with clang and the passed -Wa,-mpower4 option. But we have now removed
it expecting the compiler to automatically pass the proper options and
instructions based on -mcpu=power4.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229122521.762431-5-mpe@ellerman.id.au