Add support for Packet MMAP #3884

Job	Run time
Check --no-default-features (nightly)	24s
Check (stable)	1m 16s
Check nightly-only targets (nightly)	25s
Check (nightly)	1m 8s
Check selected Tier 3 platforms (nightly)	32s
Check (1.63)	1m 25s
Rustfmt	13s
Test rustix_use_experimental_asm (powerpc64le-linux)	49s
Test use-libc (ubuntu)	31s
Test use-libc (i686-linux)	49s
Test use-libc (aarch64-linux)	44s
Test (ubuntu)	29s
Test use-libc (powerpc64le-linux)	47s
Test use-libc (riscv64-linux)	43s
Test (ubuntu-20.04)	24s
Test use-libc (arm-linux)	48s
Test (i686-linux)	32s
Test (aarch64-linux)	44s
Test (powerpc64le-linux)	44s
Test (riscv64-linux)	39s
Test (s390x-linux)	41s
Test (arm-linux)	36s
Test (ubuntu-stable)	25s
Test (i686-linux-stable)	36s
Test (aarch64-linux-stable)	36s
Test (riscv64-linux-stable)	40s
Test (s390x-linux-stable)	41s
Test (powerpc64le-linux-stable)	40s
Test (arm-linux-stable)	42s
Test (ubuntu-1.63)	43s
Test (i686-linux-1.63)	44s
Test (aarch64-linux-1.63)	46s
Test (riscv64-linux-1.63)	29s
Test (s390x-linux-1.63)	41s
Test (powerpc64le-linux-1.63)	35s
Test (arm-linux-1.63)	44s
Test (macos-latest)	41s
Test (macos-11)	41s
Test (windows)	29s
Test (windows-2019)	31s
Test (musl)	43s
	27m 30s

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