[PowerPC64LE] Use a shared LIBC_ARCH for powerpc64le.

Given that we have converted to ELFv2 for BE already, endianness is the only
difference between the two ARCHs.

As such, there is no need to differentiate LIBC_ARCH between the two.

Combining them like this lets us avoid needing to have two copies of several
bits for no good reason.

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