libcxx: respond to some feature macro changes in LLVM 20

https://github.com/llvm/llvm-project/commit/ba87515fea90b5d55836a8e3be63a7e683ce299d

closes #25376
4fd78f9c26Alex Rønne Petersen committed on 9/28/2025, 1:31:22 PMparentebaec8e
1 file changedLine totals unavailable