Allow CP10/CP11 operations on ARMv5/v6

Those registers are VFP/NEON and vector instructions should be used instead,
but old cores rely on those co-processors to enable VFP unwinding. This change
was prompted by the libc++abi's unwinding routine and is also present in many
legacy low-level bare-metal code that we ought to compile/assemble.

Fixing bug PR20025 and allowing PR20529 to proceed with a fix in libc++abi.

llvm-svn: 214802