iris: Fill out compute caps and enable clover support

iris: Fill out compute caps and enable clover support

This commit enables clover support for iris.  It is intended as a
compiler developer tool and not as a new OpenCL implementation from
Intel.  If you want competent OpenCL, we have a different open-source
driver for that built on our LLVM-based IGC compiler stack.  However,
using clover with iris is becoming increasingly useful as a compiler
development tool and I'm getting tired of carrying the patches in a
private branch.

By default, clover will not initialize on iris.  To enable clover, set
the IRIS_ENABLE_CLOVER environment variable to "1" or "true".  As we've
done with the semi-sketchy platform support in ANV, it dumps a very loud
WARNING to stderr when enabled.  Use at your own risk.

NOTE: To anyone intending to benchmark this, the performance is going to
be terrible and that is expected.  This is in no way representative of
the Intel/NIR compiler stack.  As it currently stands, clover passes
-O0 to clang when compiling OpenCL C to make SPIRV-LLVM-Transator work.
When compiling the SPIR-V, clover currently doesn't run any NIR
optimizations before it lowers memory access so any NIR optimizations
iris attempts to do are severely hampered.  One day, clover will get a
NIR optimization loop or the ability to hand things off to the driver
per-lowering but today is not that day.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7047>