Remove dead definitions of NATGCC and CP, and dead .tll, .tbc rules.

Make should continue even if compilation cmds fail, for the sake of
the nightly tester, so use minuses on them.

Use LLVMAS, LLVMGCC, LLVMGXX instead of LAS, LCC, LCXX (as per FIXME).

llvm-svn: 10825