[PECOFF] Invoke cvtres.exe in the driver.

Previously we invoked cvtres.exe for each compiled Windows
resource file. The generated files were then concatenated
and embedded to the executable.

That was not the correct way to merge compiled Windows
resource files. If you just concatenate generated files,
only the first file would be recognized and the rest would
be ignored as trailing garbage.

The right way to merge them is to call cvtres.exe with
multiple input files. In this patch we do that in the
Windows driver.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@212763 91177308-0d34-0410-b5e6-96231b3b80d8