powerpc: Reserve correct scratch region size below stack

According to the ELF ABI v2, two scratch regions are reserved below the stack
pointer, one 288 byte general region and one 224 byte compiler region.

FreeBSD only reserved the 288 byte region. Follow the ELV v2 ABI and reserve
the full 512 byte region as specified.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>

MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1756