Fix core files for 32 bit architectures that are supported in ProcessELFCore.cpp

Core files need to know the size of the PRSTATUS header so that we can grab the register values that follow it. The code that figure out this size was using a hard coded list of architecture cores instead of relying on 32 or 64 bit for most cores.

The fix here fixes core files for 32 bit ARM. Prior to this the PRSTATUS header size was being returned as zero and the register values were being taken from the first bytes of the PRSTATUS struct (signo, etc).

Differential Revision: https://reviews.llvm.org/D58985

llvm-svn: 355526