powerpc/85xx: Remove -Wa,-me500

This caused lwsync to be converted to sync on 64-bit (on 32-bit lwsync
is generated at runtime, and so wasn't affected).  Not using lwsync
caused a significant slowdown on certain workloads.

Setting this flag for any e500-enabled build is also not friendly to
multiplatform kernels.

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