Force use of portable blst in nodejs bindings

[jtraglia]

• Add __BLST_PORTABLE__ to list of defines for all source files/platforms.

Note, there was this warning in the build step:

ld: warning: tentative definition of '___blst_platform_cap' with size 4 from 'Release/obj.target/kzg/deps/blst/build/assembly.o' is being replaced by real definition of smaller size 0 from 'Release/obj.target/kzg/deps/blst/src/server.o'

But I think this is fine. Probably a side-effect of blst's implementation. In blst, there is a tentative symbol (aka extern) defined in the assembly file. Then the real variable is defined in C code. Typically, when you list symbols there are two:

$ nm libblst.a | ag cap
0000000000000004 C ___blst_platform_cap
00000000000201b8 S ___blst_platform_cap

I think nodejs is deleting the tentative symbol/variable and just keeping the real one.

$ nm kzg.node | ag cap
0000000000034240 s ___blst_platform_cap

Also notice that S is now lowercase s which I think means it's a local variable.

I made sure that runtime detection was working by:

• modifying the optimized asm code for sha256 to an illegal instruction,
• changing ___blst_platform_cap to a different value,
• check to see that it doesn't crash (because it's using the portable impl),
• reverting ___blst_platform_cap,
• check that it does crash (because it's using the optimized impl).

To me this checks out, but something we could/should investigate more in the future.

[asn-d6]

Thanks for the investigation! LGTM!