Add an option to use global variable as the stack protector cookie.

Change-Id: I4b9d179b936aceace65b2fa46e81b9b7971ea59e
TEST: Add ARM TARGET2 relocation.

According to ARM EHABI, we should use TARGET2 as the
relocation type for type info reference, so we have
to add R_ARM_TARGET2 to MCExpr.

This patch is backported from upstream r167633
by Anton Korobeynikov.

Change-Id: I957fd1c43e0aec7c4b361ad29930fe4358ea610e

Conflicts:

	lib/MC/MCExpr.cpp