Optional offloading to AMD GPUs

[iotamudelta]

This initial work via opt-in configure option enables offloading of some sgemm, dgemm, cgemm, zgemm operations to AMD GPUs via AMD's rocBLAS. It hence requires a working ROCm software stack and ROCm-enabled accelerator.

After enabling offloading capability, the default is "never offload". Offloading can be controlled through three environment variables:
BLIS_OFFLOAD=[never,always,thresh] - thresh enables threshold-dependent offloading
BLIS_OFFLOAD_SGEMM_THRESH=$number1 the threshold of MN size of sgemm after which offloading should be attempted - must be specified
BLIS_OFFLOAD_DGEMM_THRESH=$number2 the threshold of MN size of dgemm after which offloading should be attempted - must be specified
BLIS_OFFLOAD_CGEMM_THRESH=$number3 the threshold of MN size of cgemm after which offloading should be attempted - must be specified
BLIS_OFFLOAD_ZGEMM_THRESH=$number4 the threshold of MN size of zgemm after which offloading should be attempted - must be specified

Currently known limitations:

• offloading decision is made purely based on M*N size of gemm in conjunction w/ user-controlled thresholds (or always/never offload)
• rocBLAS is initialized w/ default settings - it'll hence use the first enumerated accelerator in a system and default stream

Future work:

• offloading of integer gemms can be supported
• better offloading decision engine and performance model with less user input required

[jeffhammond]

Why not make it a draft commit if you don't want it merged?

[iotamudelta]

@jeffhammond should be ready for merge soon after WIP items done - and I'm happy to get any functional reviews already.