powerpc/e500: Set -mcpu flag for 32-bit e500

Unlike 64-bit, we don't currently support multiplatform between e500
and non-e500, so the -mcpu is not configurable at this time.

-msoft-float is specified when testing for -mcpu=8540 because otherwise
some older toolchains will fail with "error: E500 and FPRs not
supported".

Signed-off-by: Scott Wood <scottwood@freescale.com>