Added Mapping Symbols for ARM ELF

Before this patch, when you objdump an LLVM-compiled file, objdump tried to
decode data-in-code sections as if they were code.  This patch adds the missing
Mapping Symbols, as defined by "ELF for the ARM Architecture" (ARM IHI 0044D).

Patch based on work by Greg Fitzgerald.

llvm-svn: 169609