[gn build] (manually) merge a whole bunch of libc++ header files

I noticed __availability was missing, so I manually diffed the
file lists and put all recently(ish) added headers:
* __availability from 2eadbc86
* concepts from 601f7631
* execution from 0a06eb91
* numbers from 4f6c4b47

Also remove libcxx_install_support_headers like the CMake build did in
6706342f, and unconditionally copy
support/win32/{limits_msvc_win32.h,locale_win32.h} like the CMake
build always did as far as I can tell.