Added MIPS 32-bit big-endian

[DavidHorton5339]

Now that ring has generic fallback code, we can compile it for new targets by allowing them through target.h checks.
I'd like to add MIPS support. This works well (all unit tests pass, and our application works on the target).
Unfortunately the Rust compiler recently got broken for MIPS, so version 1.71 or earlier is required.
Same as #1859, but without the unsquashable multiple commits over > 1 branch.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c3fda8b) 96.00% compared to head (6ce9f07) 96.02%.
Report is 103 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1894      +/-   ##
==========================================
+ Coverage   96.00%   96.02%   +0.01%     
==========================================
  Files         138      136       -2     
  Lines       20746    20776      +30     
  Branches      226      226              
==========================================
+ Hits        19918    19950      +32     
- Misses        790      792       +2     
+ Partials       38       34       -4     

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[briansmith]

My understanding is that this target basically wouldn't work in Rust 1.72 or later without a self-built toolchain? Is that right?

[briansmith]

@DavidHorton5339 I am closing this in favor of #1910, which rebases this PR on top of the latest changes to main and adds this and mipsel to the CI test matrix.