fix(c): invalidate cache for .include and .incbin directives

[bleggett]

This fixes a really nasty bug with sccache for C code where .incbin and .include directives weren't fully checked for cache consistency before being returned in the build cache.

This led to a case where a Linux kernel build that used sccache ended up with kmods signed with the wrong signing key, because the kernel's certs/system_certificates.S embeds the cert via assembler .incbin, and sccache would mistakenly return a stale signing key from a previous build 🙃

Ex:
certs/system_certificates.S embeds the cert via assembler .incbin:

  asm
  __module_cert_start:
        .incbin "certs/signing_key.x509"
  __module_cert_end:
        .incbin "certs/x509_certificate_list"

The sizes come from label arithmetic, not from the source text. So the preprocessed text of that file is byte-identical on every build regardless of which key is in signing_key.x509. sccache hashes the preprocessed output, gets a hit, and hands back an object containing the old certificate.

ccache refuses to cache any translation unit containing .incbin precisely because of this (ccache/ccache#136, documented in the ccache manual https://ccache.dev/manual/4.13.6.html).

sccache has the detector but only wires it into preprocessor cache mode, which isn't enough.

Tested with a kernel build using this SHA, mismatch issue goes away. Also added some more unit tests.

I also did a pass to see if there are other similarly-shaped problems where the cache hashing/keying might be broken and return the wrong thing and found a few more minor ones, mostly around plugins/specs/struct layout randomization, those are fixed in the second commit.

[AJIOB]

This also should fix #2700

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.14388% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.25%. Comparing base (46e96ab) to head (0bb81ee).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/c.rs	91.50%	22 Missing ⚠️
src/compiler/gcc.rs	94.23%	3 Missing ⚠️
src/compiler/compiler.rs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2795      +/-   ##
==========================================
+ Coverage   72.90%   73.25%   +0.35%     
==========================================
  Files          72       72              
  Lines       37275    37819     +544     
==========================================
+ Hits        27176    27705     +529     
- Misses      10099    10114      +15     

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[AJIOB]

Hi @bleggett ,

Looks like you have conflicts. Please, rebase your changes

[bleggett]

Hi @bleggett ,

Looks like you have conflicts. Please, rebase your changes

Rebased, thanks!

[AJIOB]

Thanks for the fixes, @bleggett

Is it ok for you, @sylvestre ?