Enable build.rs to build each source file in parallel

[NobodyXu]

This PR enables build.rs to run perl and nasm and cc in parallel to speedup compilation.
Parallel execution of perl and nasm is done manually while parallel execution of cc is done using cc::Build.

This PR also builds src and test in parallel.

[NobodyXu]

On my powerful M1, this reduces compilation time from 4.3s => 4.2s, but on CI and other env with a much weaker CPU and much less ram, the reduction can be more significant.

I also submit a PR to cc to speedup parallel compilation, would also help ring once it is merged: rust-lang/cc-rs#780

[briansmith]

Let's split this up into multiple PRs.

parallel execution of cc is done using cc::Build.

PR #1699 will make it so that on non-Windows targets, cc::Build can build all .c and all .S files in a library in parallel, if we enable its parallel build feature.

perl

Let's do this last. Most users of ring will be building it from the crates.io release that packages the output of the PerlAsm so that perl doesn't need to be run at all.

nasm

Again, most users will be using the crates.io release where the nasm output has been prepackaged into the crate, so this isn't as urgent as the rest.

This PR also builds src and test in parallel.

I think issue #1705 describes a better solution for this.

WDYT?

[NobodyXu]

This PR also builds src and test in parallel.

I think issue #1705 describes a better solution for this.

WDYT?

I think #1705 is indeed a better solution.
Closing this PR.