[libc++abi] Implement __cxa_init_primary_exception and use it to optimize...

[libc++abi] Implement __cxa_init_primary_exception and use it to optimize std::make_exception_ptr (#65534)

This patch implements __cxa_init_primary_exception, an extension to the 
Itanium C++ ABI. This extension is already present in both libsupc++ and 
libcxxrt. This patch also starts making use of this function in 
std::make_exception_ptr: instead of going through a full throw/catch 
cycle, we are now able to initialize an exception directly, thus making 
std::make_exception_ptr around 30x faster.