Wrap declarations when including from C++

[gsauthof]

Motivation: Previously, one had to remember to always include SDK
headers from a C++ translation unit like this:

extern "C" {
#include <gd32vf103_rcu.h>
#include <gd32vf103_gpio.h>
}

Which arguably is a bit tedious and errorprone. And when leaving those
out it's perhaps not entirely obvious to beginners why they are getting
weird linker errors.

This commit thus wraps all the SDK API headers inside appropriate guards
using with the following macros

GD32VF103_BEGIN_DECLS
GD32VF103_END_DECLS

such that a C++ translation can include them simply via e.g.:

#include <gd32vf103_rcu.h>
#include <gd32vf103_gpio.h>

Note that this pattern follows what other libraries such as the Glibc,
Glib or GSL do with their headers.

[fanghuaqi]

LGTM, I will merge it when CI passed