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