[ELF] Minimal implementation for ARM static linking

The code is able to statically link the simplest case of:

  int main() { return 0; }

* Only works with ARM code - no Thumb code, no interwork (-marm -mno-thumb-interwork)
* musl libc built with no interwork and no Thumb code

Differential Revision: http://reviews.llvm.org/D6716

From: Denis Protivensky <dprotivensky@accesssoftek.com>
llvm-svn: 226643