Update build glue for LLVM/Clang 3.6svn r215315. Changes include various
performance improvements across all backends and a much improved PPC32
backend.

Update LLVM/Clang snapshot to r164464. This adopts the GCC options for
the SSP parameters and a not-yet-default rewrite of SROA.

Add LLVM and clang build system. It is enabled by MKLLVM=yes.
Upstream sources can be fetched by running "make checkout" in
src/external/bsd/llvm, they will be properly imported once the
integration

Add LLVM and clang build system. It is enabled by MKLLVM=yes.
Upstream sources can be fetched by running "make checkout" in
src/external/bsd/llvm, they will be properly imported once the
integration and missing features are sorted out.

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