add realpath.c to `LIBC_TOP_HALF_MUSL_SOURCES` #1101

Job	Run time
wasi-headers test	0s
Rustfmt	5s
Build libc (ubuntu-latest, 10.0.0)	1m 29s
Build libc (macos-latest, 10.0.0)	3m 5s
Build libc (windows-latest, 10.0.0)	2m 9s
Build libc (ubuntu-latest, 16.0.0, x86_64-linux-gnu-ubuntu-18.04)	2m 52s
Build libc (macos-latest, 15.0.7, x86_64-apple-darwin21.0)	2m 48s
Build libc (windows-latest, 16.0.0)	3m 8s
	15m 36s

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