arm/SYS.h: align with other arches

[brooks]

Summary

Rename SYSTRAP() macro to _SYSCALL() and add _SYSCALL_BODY() which invokes
the syscall via _SYCALL() and then calls cerror as required. Use to
implement PSEUDO() and RSYSCALL() removing _SYSCALL_NOERROR().

Sponsored by: DARPA

[brooks]

This is part of a set of changes I've posted at https://github.com/brooksdavis/freebsd/tree/SYS.h-unification. This compiles but I've not been able to test it.

[brooks]

With this change, arm is still the odd one out with all the __CONCAT() use, but it's much more closely aligned than before.

[brooks]

I've done basic smoke testing of this patch in a jail on an aarch64 system