misc: Depend on 'cshim', if available, instead of host freestanding hdrs #3392

Job	Run time
Compile sysdeps (dripos)	2m 24s
Compile sysdeps (lemon)	2m 29s
Build mlibc (x86_64, mlibc)	3m 19s
Compile sysdeps (aero)	2m 29s
Build mlibc (x86_64, mlibc-static)	4m 9s
Compile sysdeps (ironclad)	2m 30s
Compile sysdeps (lyre)	2m 8s
Build mlibc (x86_64, mlibc-shared)	2m 50s
Build mlibc (x86_64, mlibc-ansi-only)	2m 58s
Compile sysdeps (keyronex)	2m 44s
Build mlibc (x86_64, mlibc-headers-only)	1m 50s
Build mlibc (riscv64, mlibc)	3m 15s
Build mlibc (riscv64, mlibc-static)	4m 17s
Build mlibc (riscv64, mlibc-shared)	2m 45s
Build mlibc (riscv64, mlibc-ansi-only)	2m 36s
Build mlibc (riscv64, mlibc-headers-only)	1m 52s
Build mlibc (aarch64, mlibc)	3m 35s
Build mlibc (aarch64, mlibc-static)	2m 56s
Build mlibc (aarch64, mlibc-shared)	3m 25s
Build mlibc (aarch64, mlibc-ansi-only)	2m 50s
Build mlibc (aarch64, mlibc-headers-only)	1m 51s
Build mlibc (x86, mlibc)	4m 7s
Build mlibc (x86, mlibc-static)	3m 2s
Build mlibc (x86, mlibc-shared)	3m 41s
Build mlibc (x86, mlibc-ansi-only)	3m 42s
Build mlibc (x86, mlibc-headers-only)	2m 44s
	1h 16m 28s

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