Move special va_list handling to kmp_os.h

Instead of copying and pasting the same #ifdef expressions in
multiple places, define a type and a pair of macros in kmp_os.h, to
handle whether va_list is pointer-like or not:

* kmp_va_list is the type to use for __kmp_fork_call()
* kmp_va_deref() dereferences a va_list, if necessary
* kmp_va_addr_of() takes the address of a va_list, if necessary

Also add FreeBSD to the list of OSes that has a non pointer-like
va_list. This can now be easily extended to other OSes too.

Reviewed By: AndreyChurbanov

Differential Revision: https://reviews.llvm.org/D86397