fix hwcap access in powerpc-sf setjmp/longjmp

fix hwcap access in powerpc-sf setjmp/longjmp

commit 7be59733d71ada3a32a98622507399253f1d5e48 introduced the
hwcap-based branches to support the SPE FPU, but wrongly coded them as
bitwise tests on the computed address of __hwcap, not a value loaded
from that address. replace the add with indexed load to fix it.