[libc++] Use generator expression to simplify the CMake code

A comment was left for when we would require CMake >= 3, which we do now.
I expect this should be a NFC.

Differential Revision: https://reviews.llvm.org/D97341

GitOrigin-RevId: 5034d7115df4fb07ab08cfaf6a671f6601c28408
1 file changed