[compiler-rt] Simple crtbegin.o and crtend.o implementation

Clang relies on existence of certain symbols that are normally
provided by crtbegin.o/crtend.o. However, LLVM does not currently
provide implementation of these files, instead relying on either
libgcc or implementations provided as part of the system.

This change provides an initial implementation of crtbegin.o/crtend.o
that can be used on system that don't provide crtbegin.o/crtend.o as
part of their C library.

Differential Revision: https://reviews.llvm.org/D28791

llvm-svn: 359576