cpufeatures: add compile time fallback for unsupported targets and target features #854

Job	Run time
build (stable, thumbv7em-none-eabi)	14s
build (1.85.0, thumbv7em-none-eabi)	18s
cross (aarch64-unknown-linux-gnu, 1.85.0)	45s
build (stable, wasm32-unknown-unknown)	14s
x86 (x86_64-unknown-linux-gnu, stable)	14s
build (1.85.0, armv7a-none-eabi)	17s
build (1.85.0, wasm32-unknown-unknown)	18s
build (stable, armv7a-none-eabi)	17s
x86 (x86_64-unknown-linux-gnu, 1.85.0)	23s
test (x86_64-apple-darwin, macos-latest, 1.85.0)	20s
test (x86_64-pc-windows-msvc, windows-latest, 1.85.0)	2m 15s
minimal-versions / minimal-versions	56s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	15s
test (x86_64-pc-windows-msvc, windows-latest, stable)	2m 37s
cross (powerpc-unknown-linux-gnu, 1.85.0)	44s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.85.0, sudo apt update && sudo apt install gcc-mult...	57s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	28s
cross (aarch64-unknown-linux-gnu, stable)	50s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.85.0)	19s
cross (powerpc-unknown-linux-gnu, stable)	45s
test (x86_64-apple-darwin, macos-latest, stable)	21s
	13m 47s

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