[docs] Nuke some references to llvm-gcc

From a cursory look it seems like all the described commandline options
and such apply to clang just fine, but I'd appreciate a second opinion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201616 91177308-0d34-0410-b5e6-96231b3b80d8