feat: FFI_PORTABLE to build a maximally portable binary

[rvagg]

Ref: filecoin-project/lotus#12423

Background: we're having trouble building from source in Lotus CI because we run tests two different machine types and only build FFI on one, caching the output and deploying it on the other machines. We have our own custom runners in AWS and also lean heavily on the default GHA runners which are in Azure and they obviously have different CPU types.

This change introduces FFI_PORTABLE=1 which builds using the same flags as rust/scripts/build-release.sh does when called from CI to build the release bundle.

Comparison of some builds:

When run from CI, we get:

+ RUSTFLAGS='--print native-static-libs '
+ cargo build --release --locked --verbose --no-default-features --features multicore-sdr,opencl,blst-portable

When run from FFI_BUILD_FROM_SOURCE=1 make (on my machine), we get:

+ RUSTFLAGS='--print native-static-libs -C target-feature=+adx,+sha,+sse2,+avx2,+avx,+sse4.2,+sse4.1'
+ cargo build --release --locked --no-default-features --features multicore-sdr,cuda

FFI_BUILD_FROM_SOURCE=1 FFI_USE_BLST_PORTABLE=1 make gives:

+ RUSTFLAGS='--print native-static-libs -C target-feature=+adx,+sha,+sse2,+avx2,+avx,+sse4.2,+sse4.1'
+ cargo build --release --locked --no-default-features --features multicore-sdr,blst-portable,cuda

And this new option, FFI_BUILD_FROM_SOURCE=1 FFI_PORTABLE=1 make, gives:

+ RUSTFLAGS='--print native-static-libs '
+ cargo build --release --locked --no-default-features --features multicore-sdr,blst-portable,opencl

i.e. the same to the CI version except for --verbose.