Add the riscv_ext_intrinsics feature for #114544

[coastalwhite]

This PR starts the implementation of #114544.

This feature is needed to remove the feature gate on stdsimd for the RISC-V intrinsics.

[rustbot]

r? @b-naber

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[rust-log-analyzer]

The job x86_64-gnu-llvm-15 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

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...........................................................................

failures:

---- [ui] tests/ui/feature-gates/feature-gate-riscv-ext-intrinsics.rs stdout ----
error: ui test compiled successfully!
status: exit status: 0
status: exit status: 0
command: RUSTC_ICE="0" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/feature-gates/feature-gate-riscv-ext-intrinsics.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/feature-gates/feature-gate-riscv-ext-intrinsics" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/feature-gates/feature-gate-riscv-ext-intrinsics/auxiliary"
stderr: none




failures:
    [ui] tests/ui/feature-gates/feature-gate-riscv-ext-intrinsics.rs
test result: FAILED. 15248 passed; 1 failed; 138 ignored; 0 measured; 0 filtered out; finished in 133.80s

Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
Build completed unsuccessfully in 0:10:53

[coastalwhite]

Heya! I am a bit unfamiliar with this process and I cannot find exactly how to do this. I tried following this https://rustc-dev-guide.rust-lang.org/implementing_new_features.html, but this is for language features and not core lib additions.

I am also failing to find another example of people using this process. Is this even needed for a library feature?

[coastalwhite]

Closing as this does not seem necessary anymore.