arm/SYS.h: align with other arches

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().

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43061