Added functions to the C++ API, for the benefit of non-8-bit byte architectures.

New functions to give client applications to tools to discover target byte sizes
for addresses prior to ReadMemory. Also added GetPlatform and ReadMemory to the
SBTarget class, since they seemed to be useful utilities to have.

Each new API has had a test case added.

http://reviews.llvm.org/D5867

llvm-svn: 220372