MC, COFF: Use relocations for function references inside the section

Referencing one symbol from another in the same section does not
generally require a relocation.  However, the MS linker has a feature
called /INCREMENTAL which enables incremental links.  It achieves this
by creating thunks to the actual function and redirecting all
relocations to point to the thunk.

This breaks down with the old scheme if you have a function which
references, say, itself.  On x86_64, we would use %rip relative
addressing to reference the start of the function from out current
position.  This would lead to miscompiles because other references might
reference the thunk instead, breaking function pointer equality.

This fixes PR21520.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221678 91177308-0d34-0410-b5e6-96231b3b80d8
Was convinced in commit comments that requiring a specific python version is the wrong approach; reverting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221679 91177308-0d34-0410-b5e6-96231b3b80d8