fix: Always generate .ra CFI on MIPS

[Swatinem]

this is just a hack so far, it needs to be cleaned up so its more reliable, and obviously should only be used when we are dealing with MIPS.

[codecov-commenter]

Codecov Report

Merging #612 (61e3e3a) into master (8c11b8a) will increase coverage by 0.05%.
The diff coverage is 94.59%.

❗ Current head 61e3e3a differs from pull request most recent head 3df29d9. Consider uploading reports for the commit 3df29d9 to get more accurate results

@@            Coverage Diff             @@
##           master     #612      +/-   ##
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+ Coverage   68.26%   68.32%   +0.05%     
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  Files          83       83              
  Lines       16434    16478      +44     
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+ Hits        11219    11258      +39     
- Misses       5215     5220       +5     

[loewenheim]

Context: both Breakpad and rust-minidump expect there to always be an unwind rule for the return address or they will reject the CFI. On MIPS, however, the DWARF doesn't always contain such a rule because the return address is simply saved in its own register. rust-minidump/rust-minidump#583 (comment) suggests several ways to deal with this problem; this PR implements option 3.

[Swatinem]

Now that our stackwalker does reasonably work, I tried again with and without this default, and in the very limited test data that I have, indeed the context frames are the ones that have CFI (and potentially a stack frame), but no .ra.