Test suite runs better again after recent fixes that would select a platform...

Test suite runs better again after recent fixes that would select a platform if a "file a.out" auto selected a different platform than the selected one.

Changes include:
- fix it so you can select the "host" platform using "platform select host"
- change all callbacks that create platforms to returns shared pointers
- fix TestImageListMultiArchitecture.py to restore the "host" platform by running "platform select host"
- Add a new "PlatformSP Platform::Find(const ConstString &name)" method to get a cached platform
- cache platforms that are created and re-use them instead of always creating a new one

llvm-svn: 218145