internal/cpu: add experiment to disable CPU features with GODEBUGCPU
Needs the go compiler to be build with GOEXPERIMENT=debugcpu to be active.
The GODEBUGCPU environment variable can be used to disable usage of
specific processor features in the Go standard library.
This is useful for testing and benchmarking different code paths that
are guarded by internal/cpu variable checks.
Use of processor features can not be enabled through GODEBUGCPU.
To disable usage of AVX and SSE41 cpu features on GOARCH amd64 use:
GODEBUGCPU=avx=0,sse41=0
The special "all" option can be used to disable all options:
GODEBUGCPU=all=0