Generalize bugpoint's concept of a "safe" backend, and add options

Generalize bugpoint's concept of a "safe" backend, and add options
to allow the "safe" backend to be run with a different path, and/or
with different command-line options.

This enables the following use cases:
 - bugpoint llc against an llc command from a different build
 - bugpoint llc against the same llc with different command-line options
 - and more...

Also, document the existing "custom" interpreter options.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60681 91177308-0d34-0410-b5e6-96231b3b80d8