compilersupport: add support for C23's `nullptr` #1

Job	Run time
ubuntu-latest/linux-g++	1m 2s
ubuntu-latest/gcc-freestanding	59s
ubuntu-latest/clang	52s
ubuntu-latest/clang-small	1m 10s
ubuntu-latest/gcc-small	56s
macos-13/clang	1s
ubuntu-latest/gcc-no-math	58s
macos-13/clang-small	1s
	5m 59s

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