Implement `runtime::brk`. (#891) #3467

Job	Run time
Check (stable)	8m 12s
Check (nightly)	8m 19s
Check selected Tier 3 platforms (nightly)	4m 57s
Check nightly-only targets (nightly)	29s
Check --no-default-features (nightly)	1m 17s
Check (1.63)	15m 57s
Rustfmt	9s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 31s
Test use-libc (ubuntu)	55s
Test (ubuntu)	1m 5s
Test use-libc (i686-linux)	2m 9s
Test (ubuntu-20.04)	1m 2s
Test use-libc (aarch64-linux)	2m 11s
Test (i686-linux)	1m 26s
Test use-libc (powerpc64le-linux)	1m 40s
Test (aarch64-linux)	1m 36s
Test (powerpc64le-linux)	1m 48s
Test (riscv64-linux)	1m 46s
Test use-libc (riscv64-linux)	1m 23s
Test (s390x-linux)	1m 53s
Test use-libc (arm-linux)	1m 40s
Test (arm-linux)	1m 32s
Test (ubuntu-stable)	58s
Test (i686-linux-stable)	1m 32s
Test (aarch64-linux-stable)	1m 54s
Test (riscv64-linux-stable)	1m 26s
Test (s390x-linux-stable)	1m 21s
Test (powerpc64le-linux-stable)	1m 57s
Test (arm-linux-stable)	1m 24s
Test (ubuntu-1.63)	5m 34s
Test (i686-linux-1.63)	5m 57s
Test (aarch64-linux-1.63)	6m 0s
Test (riscv64-linux-1.63)	5m 53s
Test (s390x-linux-1.63)	6m 16s
Test (powerpc64le-linux-1.63)	6m 50s
Test (arm-linux-1.63)	6m 44s
Test (macos-latest)	1m 32s
Test (macos-11)	1m 15s
Test (windows)	1m 17s
Test (windows-2019)	1m 9s
	2h 1m 56s

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