[llvm-c] Simplify signature of LLVMGetTargetFromName

LLVMGetTargetFromName was not yet present in an LLVM release,
so this does not break compatibility.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194769 91177308-0d34-0410-b5e6-96231b3b80d8
[llvm-c] Add missing const qualifiers to LLVMCreateTargetMachine

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194770 91177308-0d34-0410-b5e6-96231b3b80d8