i#1551 port to ARM: syscall mangling

Share code with x86 in mangle_shared.c, changing the arch-specific routine
to mangle_shared_arch().

For ARM inlined syscalls, we assume we don't have to restore the stolen reg
as it's higher than any syscall param or number.  Plus we assume
callee-saved regs will survive.  If the stolen reg is caller-saved, we swap
it w/ a callee-saved reg.

Review-URL: https://codereview.appspot.com/183640043