[libc++] Move checks for newlib to actually work

The checks did not work in __config, since no header defining
`_NEWLIB_VERSION` was included before. This patch moves the two
checks for newlib to the headers that actually need it - and after
they already include relevant headers.

Differential Revision: https://reviews.llvm.org/D79888

GitOrigin-RevId: 8deaa4a1471dac19c717a320eec9956843a8c479
3 files changed