Generalize the EXPORTED_SYMBOL_FILE concept in the Makefiles to work with

Generalize the EXPORTED_SYMBOL_FILE concept in the Makefiles to work with
native linking export files, including running sed to prepend underscores
on darwin, and make use of it in libLTO and libEnhancedDisassembly.

Remove the leading underscores from library export files so that they
work with the new EXPORTED_SYMBOL_FILE support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101399 91177308-0d34-0410-b5e6-96231b3b80d8