unify non-inline version of syscall code across archs

unify non-inline version of syscall code across archs

except powerpc, which still lacks inline syscalls simply because
nobody has written the code, these are all fallbacks used to work
around a clang bug that probably does not exist in versions of clang
that can compile musl. however, it's useful to have the generic
non-inline code anyway, as it eases the task of porting to new archs:
writing inline syscall code is now optional. this approach could also
help support compilers which don't understand inline asm or lack
support for the needed register constraints.

mips could not be unified because it has special fixup code for broken
layout of the kernel's struct stat.