toolchain: add powerpc SPE ABI support

Add the ability for buildroot to build an SPE ABI enabled toolchain.

This is mandatory for e500v1/v2 cores since they don't support classic
FPU mode as the e500mc does.

Useful for Freescale's PowerQUICC III and single/dual-core QorIQ
line of processors.

The new TARGET_ABI variable is used rather than TARGET_CFLAGS for
uclibc's UCLIBC_EXTRA_CFLAGS to avoid breakish CFLAGS leaking in, a
good example being -mthumb for ARM.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>