Update optimization level recommendation

[jtraglia]

Instead of -O -D__BLST_PORTABLE__, I think the recommendation should be -O2 -D__BLST_PORTABLE__.

blst/bindings/go/README.md

Line 18 in 3dd0f80

If the test or target application crashes with an "illegal instruction" exception [after copying to an older system], rebuild with `CGO_CFLAGS` environment variable set to <nobr>`-O -D__BLST_PORTABLE__`</nobr>. Don't forget <nobr>`-O`</nobr>!

While blst doesn't really benefit from -O2 because it's mostly assembly, this affects the whole Golang project that uses blst. The default value is -O2 (check with go env) and changing it to -O (-O1) could cause projects to be slower than normal.

[jtraglia]

Also, just noticed the filenames here are a little out-dated. Should be cgo_server.c and cgo_assembly.S.

blst/bindings/go/README.md

Line 16 in 3dd0f80

After running generate.py, <nobr>`go build`</nobr> and <nobr>`go test`</nobr> can be run as usual. Cgo will compile `server.c`, which includes the required C implementation files, and `assembly.S`, which includes appropriate pre-generated assembly code for the platform. To compile on Windows one has to have MinGW gcc on the `%PATH%`.

[dot-asm]

Thanks!