rust: recursively pull proc-macro deps for rust targets

aee941559 ("rust: recursively pull proc-macro dependencies as well")
had to be reverted (in a66cb97e8) because it broke Mesa cross
compilation.  This happened because a C shared library was linked with
a Rust C-ABI static library, which caused it to inherit the proc macro
dependency the Rust static library was linked with.

The right way to handle this is for only Rust targets to inherit proc
macro dependencies from static libraries they link with.  A Rust
executable, library, or whatever will need the proc macros its Rust
dependencies use, as illustrated in the test case that I've
reintroduced here.

I've verified that Mesa still cross compiles correctly with this
change.  The same failure was also identified by the "rust/21
transitive dependencies" test case, but only when cross compiling, so
it wasn't caught by CI.

Co-authored-by: Xavier Claessens <xavier.claessens@collabora.com>

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