[PowerPC64] Minor correctness fix in rstcode.

TRAP_ENTRY(0) should be TRAP_GENTRAP(0) here.

However, in practice, it doesn't matter, as the only time TRAP_ENTRY and
TRAP_GENTRAP can differ is when bridge mode is active, which is impossible
on the 64 bit kernel.

Fix it anyway in case we ever need to add a trap preamble on PPC64.