remove old clang workarounds from arm syscall implementation

remove old clang workarounds from arm syscall implementation

the register constraints in the non-clang case were tested to work on
clang back to 3.2, and earlier versions of clang have known bugs that
preclude building musl.

there may be other reasons to prefer not to use inline syscalls, but
if so the function-call-based implementations should be added back in
a unified way for all archs.