sim: watch: fix pc watchpoints on little endian host systems

sim: watch: fix pc watchpoints on little endian host systems

My change 1ac72f0659d64d6a14da862242db0d841d2878d0 ("sim: convert to
bfd_endian") subtly broke the watchpoint module on little endian host
systems.  The old code used 0 to mean "whatever the host endian is",
and while that was changed to use BFD_ENDIAN_UNKNOWN, this caller was
missed.  Since its API used an int instead of an enum, the coercion
from 0 to the BFD endian enum was silently missed, and 0 happens to
be BFD_ENDIAN_BIG.

Instead of restoring the old logic by passing in BFD_ENDIAN_UNKNOWN,
we know the right host endian at compile time, so use that directly.