Increase openblas threads defaults

[ViralBShah]

As discussed in JuliaLang/julia#14857, the default number of threads on linux is probably too low as core counts have significantly gone up. My suggestion is to make the default NUM_THREADS at compile time to be 64 on Linux (until a better fix is found), and leave it at 16 on Windows, Mac and ARM.

Relevant line here:

Yggdrasil/O/OpenBLAS/build_tarballs.jl

Line 42 in 7fcd72a

# Use 16 threads unless we're on an i686 arch:

[vchuravy]

x-ref: JuliaLang/julia#18638

[ViralBShah]

Also: OpenMathLib/OpenBLAS#2225