[sanitizer] Asm implementation of syscall() for arm32.

Summary:
These will be used in an ifunc resolver, when the binary may not be
completely relocated, and syscall() function from libc could not be
used.

Reviewers: dvyukov, vitalybuka

Subscribers: aemerson, kubamracek, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D39701

llvm-svn: 317640