Sync headers with cheriot-rtos

[davidchisnall]

CHERIoT RTOS has a subset of the libc++ headers for a freestanding (plus some optional bits) here:

https://github.com/CHERIoT-Platform/cheriot-rtos/tree/main/sdk/include/libc%2B%2B

These are still the versions from LLVM 13 (which work, but lack some newer concepts bits).

They need updating to the newer ones.

I believe the new libc++ removed the option in std::vector that allowed it to work as a header-only library. It now depends on some things in an extern template definition that should be provided by the library.

We should make sure that we define the export macros to __cheriot_libcall and build a std.vector shared library for this (and then the same pattern for other bits of libc++ that we want to use).