Reorganize the CPU feature detection

[antoyo]

Move the code from i386-rust.cc to i386-rust-and-jit.inc so that it can be reused by libgccjit.

gcc/ChangeLog:

* config/i386/i386-rust-and-jit.inc: New file.
* config/i386/i386-rust.cc: Move code to i386-rust-and-jit.inc.

Here is a checklist to help you with your PR.

• GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
• Read contributing guidlines
• make check-rust passes locally (it doesn't pass, but same number of failures as on master)
• Run clang-format
• (n/a) [ ] Added any relevant test cases to gcc/testsuite/rust/

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Since both libgccjit (via rustc_codegen_gcc) and gccrs need to do the same CPU features detection, refactor this code to avoid duplication.

[antoyo]

As discussed with you, @CohenArthur, here's the PR that will allow us to not have duplication for the CPU features detection.

Some more context on the mailing list.

I already tested this in rustc_codegen_gcc: you can see the libgccjit commit here.

Test results:

		=== rust Summary ===

# of expected passes		9158
# of expected failures		315
# of unsupported tests		8

(same on the master branch)

[antoyo]

cc @davidmalcolm: Is this what you had in mind?