Fix userland program exception handling for powerpc64.

It appears that the linker will not handle 64-bit relocations at addresses that
are not aligned to 8-byte boundaries. Prior to this change the line:

.llong generictrap

was aligned to a 4-byte address, and the linker replaced that with an 8-byte
0x0. Aligning that address to 8 bytes caused the linker to generate the proper
relocation. As a follow-through, the dblow from trap_subr33.S used the code
sequence 'lwz %r1, TRAP_GENTRAP(0)', so this reproduces the analogue of that for
64-bit.