[libc++] Fix __regex_word value when using newlib/picolibc

The ctype mask for newlib/picolibc is fully saturated, so __regex_word
has to overlap with one of the values. This commit uses the same workaround
as bionic did (uint16_t for char_class_type inside regex_traits). It
should be possible to have libc++ provide the default rune table instead,
but that will require a new mechanism to detect newlib inside __config
since the header defining the newlib/picolibc macros has not been included
yet inside __config. Doing it this way also avoids duplicating the ctype
table for newlib, reducing the global data size.

Differential Revision: https://reviews.llvm.org/D138195