xtensa: userspace: simplify syscall trampoline a bit

There is no need to do a call4 and jx. Simply do a callx4
is enough.

Also amended the now incorrect comment about how syscall
trampoline is set up. It is now a straight call4 instead of
the old 2x call4.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>