Add compiler-rt fallbacks on loongarch64-musl #743

Job	Run time
Clippy	12s
Rustfmt	7s
Test (aarch64-unknown-linux-gnu, ubuntu-latest, nightly)	1m 48s
Test (arm-unknown-linux-gnueabi, ubuntu-latest, nightly)	1m 51s
Test (arm-unknown-linux-gnueabihf, ubuntu-latest, nightly)	1m 50s
Test (i586-unknown-linux-gnu, ubuntu-latest, nightly)	1m 35s
Test (i686-unknown-linux-gnu, ubuntu-latest, nightly)	1m 36s
Test (powerpc-unknown-linux-gnu, ubuntu-latest, nightly)	1m 50s
Test (powerpc64-unknown-linux-gnu, ubuntu-latest, nightly)	1m 48s
Test (powerpc64le-unknown-linux-gnu, ubuntu-latest, nightly)	1m 48s
Test (thumbv6m-none-eabi, ubuntu-latest, nightly)	1m 26s
Test (thumbv7em-none-eabi, ubuntu-latest, nightly)	1m 37s
Test (thumbv7em-none-eabihf, ubuntu-latest, nightly)	1m 41s
Test (thumbv7m-none-eabi, ubuntu-latest, nightly)	1m 34s
Test (wasm32-unknown-unknown, ubuntu-latest, nightly)	1m 48s
Test (x86_64-unknown-linux-gnu, ubuntu-latest, nightly)	1m 30s
Test (x86_64-apple-darwin, macos-latest, nightly)	1m 45s
Test (i686-pc-windows-msvc, windows-latest, nightly)	1m 45s
Test (x86_64-pc-windows-msvc, windows-latest, nightly)	1m 42s
Test (i686-pc-windows-gnu, windows-latest, nightly-i686-gnu)	1m 43s
Test (x86_64-pc-windows-gnu, windows-latest, nightly-x86_64-gnu)	1m 42s
success	1s
	32m 39s

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