This patch augments the HIPAMD driver to allow it to target AMDGCN
flavoured SPIR-V compilation. It's mostly straightforward, as we re-use
some of the existing SPIRV infra, however there are a few notable
additions:
- we introduce an `amdgcnspirv` offload arch, rather than relying on
using `generic` (this is already fairly overloaded) or simply using
`spirv` or `spirv64` (we'll want to use these to denote unflavoured
SPIRV, once we bring up that capability)
- initially it is won't be possible to mix-in SPIR-V and concrete AMDGPU
targets, as it would require some relatively intrusive surgery in the
HIPAMD Toolchain and the Driver to deal with two triples
(`spirv64-amd-amdhsa` and `amdgcn-amd-amdhsa`, respectively)
- in order to retain user provided compiler flags and have them
available at JIT time, we rely on embedding the command line via
`-fembed-bitcode=marker`, which the bitcode writer had previously not
implemented for SPIRV; we only allow it conditionally for AMDGCN
flavoured SPIRV, and it is handled correctly by the Translator (it ends
up as a string literal)
Once the SPIRV BE is no longer experimental we'll switch to using that
rather than the translator. There's some additional work that'll come
via a separate PR around correctly piping through AMDGCN's
implementation of `printf`, for now we merely handle its flags
correctly.
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