Revert "powerpc/e500: Update compilation flags with core specific options"

This reverts commit c8db32c8.

The commit breaks the build of all my 64-bit embedded configs. It
looks like gcc-4.7.3 doesn't know about e5500. Additionally it
incorrectly does -mcpu=e5500 on a config that has both e5500 and A2
support enabled.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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