[compiler-rt][cmake][powerpc] Remove TEST_BIG_ENDIAN from base-config-ix.cmake

It's actually not safe to call TEST_BIG_ENDIAN here, since we may be
running from the builtins build (i.e builtins-config-ix) context where
TEST_COMPILE_ONLY  is set since without builtins already built we may
fail to link, and TEST_BIG_ENDIAN internally performs tests which may
fail to link without builtins.

Fortunately powerpc is the only target that uses this information here and
we actually already know the whether we are targeting the LE variant due
to earlier macro checks, so we can simply this to remove our reliance on
TEST_BIG_ENDIAN.

Reviewed By: hubert.reinterpretcast, Whitney

Differential Revision: https://reviews.llvm.org/D88608