WIP: Add support for {mipsel,riscv64gc}-unknown-linux-gnu

[briansmith]

No description provided.

[briansmith]

This is based on @LinusU's PR #1120.

These two things succeed:

mk/cargo.sh test --target=riscv64gc-unknown-linux-gnu --test=aead_tests
mk/cargo.sh test --target=mipsel-unknown-linux-gnu --test=aead_tests

But, tests that require bn_mul_mont fail because we don't have a fallback; that is what PR #1455 is about.

Also, code coverage measurement doesn't work because the compiler wasn't built with the profiler runtime support for either target:

$ RING_COVERAGE=1 mk/cargo.sh test --target=riscv64gc-unknown-linux-gnu --test=aead_tests
error[E0463]: can't find crate for `profiler_builtins`
  |
  = note: the compiler may have been built without the profiler runtime
$ RING_COVERAGE=1 mk/cargo.sh test --target=mipsel-unknown-linux-gnu --test=aead_tests
error[E0463]: can't find crate for `profiler_builtins`
  |
  = note: the compiler may have been built without the profiler runtime

[codecov]

Codecov Report

Merging #1556 (22fae8e) into main (3915942) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1556      +/-   ##
==========================================
+ Coverage   92.24%   92.26%   +0.01%     
==========================================
  Files         127      127              
  Lines       18850    18850              
  Branches      196      196              
==========================================
+ Hits        17389    17392       +3     
+ Misses       1424     1422       -2     
+ Partials       37       36       -1     

Impacted Files	Coverage Δ
crypto/poly1305/poly1305.c	97.20% <0.00%> (+0.46%)	⬆️
src/aead/less_safe_key.rs	96.89% <0.00%> (+0.77%)	⬆️
src/io/der_writer.rs	100.00% <0.00%> (+2.17%)	⬆️

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[briansmith]

Closing this in favor of #1558.