[ELF] Define __dso_handle symbol if needed

Traditionally, it has been defined in crtbegin.o, which is typically
provided by libgcc or as part of the C library on some systems. However,
but there's no principled reason for it to be there. We optionaly
define this symbol, which can be used on platforms that don't provide
__dso_handle in crtbegin.o or which don't use crtbegin.o at all.

Differential Revision: https://reviews.llvm.org/D33856

llvm-svn: 304732