eal: add timespec_get shim

eal: add timespec_get shim

C11 timespec_get() is not provided on some platforms:

* MinGW-w64 does not currently implement it [1].
* FreeBSD 11 with Clang 10.0.0 does not provide it.

Add internal shims to Windows and FreeBSD EALs.
For Windows, it can be removed after [1] is fixed.

[1]: https://sourceforge.net/p/mingw-w64/mailman/message/37224689/

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Jie Zhou <jizh@linux.microsoft.com>
Acked-by: Nick Connolly <nick.connolly@mayadata.io>