pkg/csource: remove C23 embed built-in defines #16627

Job	Run time
build	6m 43s
aux	3m 59s
dashboard	6m 45s
race	1m 57s
old	4m 1s
race_dashboard	6m 25s
gvisor	3m 59s
arch (presubmit_arch_freebsd)	4m 2s
arch (presubmit_arch_openbsd)	3m 47s
arch (presubmit_arch_windows)	3m 30s
arch (presubmit_arch_netbsd)	3m 47s
arch (presubmit_arch_darwin)	3m 53s
arch (presubmit_arch_executor)	3m 21s
arch (presubmit_arch_linux)	4m 2s
	1h 0m 11s

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