add c_unwind feature #759

Job	Run time
Clippy	17s
Rustfmt	11s
Test (aarch64-unknown-linux-gnu, ubuntu-latest, nightly)	2m 10s
Test (arm-unknown-linux-gnueabi, ubuntu-latest, nightly)	2m 0s
Test (arm-unknown-linux-gnueabihf, ubuntu-latest, nightly)	2m 4s
Test (i586-unknown-linux-gnu, ubuntu-latest, nightly)	1m 35s
Test (i686-unknown-linux-gnu, ubuntu-latest, nightly)	1m 32s
Test (powerpc-unknown-linux-gnu, ubuntu-latest, nightly)	1m 59s
Test (powerpc64-unknown-linux-gnu, ubuntu-latest, nightly)	1m 57s
Test (powerpc64le-unknown-linux-gnu, ubuntu-latest, nightly)	1m 51s
Test (thumbv6m-none-eabi, ubuntu-latest, nightly)	1m 39s
Test (thumbv7em-none-eabi, ubuntu-latest, nightly)	1m 30s
Test (thumbv7em-none-eabihf, ubuntu-latest, nightly)	1m 29s
Test (thumbv7m-none-eabi, ubuntu-latest, nightly)	1m 26s
Test (wasm32-unknown-unknown, ubuntu-latest, nightly)	1m 50s
Test (x86_64-unknown-linux-gnu, ubuntu-latest, nightly)	1m 45s
Test (x86_64-apple-darwin, macos-latest, nightly)	1m 52s
Test (i686-pc-windows-msvc, windows-latest, nightly)	2m 5s
Test (x86_64-pc-windows-msvc, windows-latest, nightly)	1m 51s
Test (i686-pc-windows-gnu, windows-latest, nightly-i686-gnu)	2m 3s
Test (x86_64-pc-windows-gnu, windows-latest, nightly-x86_64-gnu)	1m 59s
success	1s
	35m 6s

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