mesa: Remove massive kludge that worked around PR toolchain/50277.

Now that the underlying bug in ld.elf_so is fixed, the workaround is
causing harm.

XXX pullup-10

merge mesa 19.1.7 into MesaLib.old.

initial import of Mesa 19.1.7 into MesaLib.old.

import the existing Mesa 10.x sources as MesaLib.old while we
handle the upgrade to Mesa 18.x.