[gcn][patch] Add -mgpu option and plumb in assembler/linker

[Andrew Stubbs]

This patch, for the "gcn" branch, does three things:

1. Add specs to drive the LLVM assembler and linker. It requires them to 
be installed as "as" and "ld", under $target/bin, but then the compiler 
Just Works with these specs.

2. Switch to HSACO format version 2, and have the assembler auto-set the 
architecture flags from -mcpu. This means the amdphdr utility is no 
longer required.

3. Add -mgpu option and corresponding --with-gpu. I've deliberately used 
"gpu" instead of "cpu" because I want offloading compilers to be able to 
say "-mcpu=foo -foffload=-mgpu=bar", or even have the host compiler just 
understand -mgpu and DTRT.

The patch also removes the unused and unwritten "arch" and "tune" 
settings. They can be added back when useful, but the assembler requires 
a GPU name, I think, so we need that as input.

OK to commit to GCN branch?

Andrew

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