Expand 32-bit ARM test coverage

[briansmith]

See individual commit messages for details.

[briansmith]

Sadly this needs work:

• We need to compile with clang in order to get the coverage information for the C code, but clang cannot build the sha256 and x25519 assembly for arm-unknown-linux-gnueabi. We need to improve the conditional compilation for this target in the assembly and/or change our build system.
• armv7-unknown-linux-gnueabihf doesn't have profiler_builtins; see Code coverage does not work on armv7-unknown-linux-gnueabihf due to missing profiler_builtins rust-lang/rust#79640.

[codecov]

Codecov Report

Merging #1675 (38f68ed) into main (ff2cc71) will increase coverage by 0.00%.
Report is 4 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1675   +/-   ##
=======================================
  Coverage   96.10%   96.11%           
=======================================
  Files         137      137           
  Lines       20548    20548           
  Branches      217      217           
=======================================
+ Hits        19748    19750    +2     
+ Misses        762      760    -2     
  Partials       38       38           

see 2 files with indirect coverage changes

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