Curve25519: Conservatively force noinline on ADX code paths.

[briansmith]

See rust-lang/rust#116573.

[codecov]

Codecov Report

Merging #1717 (2214138) into main (29fb827) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1717      +/-   ##
==========================================
+ Coverage   96.05%   96.07%   +0.01%     
==========================================
  Files         135      135              
  Lines       20526    20526              
  Branches      217      217              
==========================================
+ Hits        19717    19720       +3     
+ Misses        771      769       -2     
+ Partials       38       37       -1     

Files	Coverage Δ
third_party/fiat/curve25519_64_adx.h	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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[RalfJung]

These functions don't seem to use any SIMD types in their signature so I don't think they are affected by rust-lang/rust#116573. But of course there might be other bugs lurking in target-feature handling...

[briansmith]

But of course there might be other bugs lurking in target-feature handling...

Exactly. The fact is, we only have two functions that cross a boundary of target-features, and we don't intend them to be inlined, and so it is easy for us to just force the no-inlining than rely on the compiler.