Update syscall handling

[Andrew Turner]

As part of my work on getting the kernel running with the EABI I
updated how syscalls are handled to use the common
syscallenter/syscallret functions. I moved the arg parsing code to
cpu_fetch_syscall_args where it belongs and removed the duplicate code
from syscall to be replaced with syscallenter and syscallret.

I've tested on my LN2410SBC but would like a review before committing.

The td->td_ar == NULL check is from the original function. Do we still
need it? ARM appears to be the only arch with this check.

Andrew

[1] http://fubar.geek.nz/files/freebsd/freebsd-arm-syscall.diff