Add support for arm64ec #851

Job	Run time
Windows AArch64	2m 16s
iOS (aarch64-apple-ios, iphoneos)	24s
Docker (aarch64-unknown-linux-gnu)	46s
iOS (x86_64-apple-ios, iphonesimulator)	24s
Docker (arm-unknown-linux-gnueabihf)	57s
Build Targets (wasm32-unknown-unknown)	14s
Build Targets (wasm32-wasi)	19s
Docker (armv7-unknown-linux-gnueabihf)	39s
Miri	2m 12s
Build Targets (x86_64-unknown-fuchsia)	19s
Docker (i586-unknown-linux-gnu)	53s
Build Targets (x86_64-fortanix-unknown-sgx)	19s
Docker (i686-unknown-linux-gnu)	56s
Build Targets (x86_64-unknown-illumos)	19s
Docker (powerpc64-unknown-linux-gnu)	41s
MSRV	3m 49s
Rustfmt	11s
Test (ubuntu-20.04, stable)	2m 27s
Docker (s390x-unknown-linux-gnu)	50s
Test (ubuntu-20.04, beta)	1m 54s
Test (ubuntu-20.04, nightly)	2m 23s
Docker (x86_64-pc-windows-gnu)	50s
Test (macos-latest, stable)	2m 22s
Docker (x86_64-unknown-linux-gnu)	31s
Test (macos-latest, nightly)	2m 41s
Docker (x86_64-unknown-linux-musl)	32s
Test (windows-latest, stable-x86_64-msvc)	1m 54s
Docker (arm-linux-androideabi)	1m 5s
Test (windows-latest, stable-i686-msvc)	2m 14s
Docker (armv7-linux-androideabi)	1m 4s
Test (windows-latest, stable-x86_64-gnu)	2m 36s
Docker (aarch64-linux-android)	1m 7s
Docker (i686-linux-android)	1m 45s
Docker (x86_64-linux-android)	1m 11s
	43m 4s

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