[libunwind][MIPS]: Add support for unwinding in N32 processes.

[libunwind][MIPS]: Add support for unwinding in N32 processes.

Summary:
N32 uses the same register context as N64.  However, N32 requires one
change to properly fetch addresses from registers stored in memory.
Since N32 is an ILP32 platform, getP() only fetches the first 32-bits
of a stored register.  For a big-endian platform this fetches the
upper 32-bits which will be zero.  To fix this, add a new
getRegister() method to AddressSpace which is responsible for
extracting the address stored in a register in memory.  This matches
getP() for all current ABIs except for N32 where it reads the 64-bit
register and returns the low 32-bits as an address.  The
DwarfInstructions::getSavedRegister() method uses
AddressSpace::getRegister() instead of AddressSpace::getP().

Reviewers: sdardis, compnerd

Reviewed By: sdardis

Differential Revision: https://reviews.llvm.org/D39074

llvm-svn: 326250