Support GNU C `(x, ## __VA_ARGS__)` omit comma.

[jwnhy]

GNU C supports a non-standard feature that allows one to omit the variadics without the additional comma.

For example,

#define pr_fmt(fmt, ...) printf(fmt, __VA_ARGS__)

pr_fmt("HELLO") will expand to printf("HELLO", ) with an additional comma ,, which is not ideal.

GNU C introduces the feature that if adds a preceding ##, the additional comma will automatically be removed.

#define pr_fmt(fmt, ...) printf(fmt, ## __VA_ARGS__)

This produces printf("HELLO").

Given that there is already some implementation in C++ 2a around ommitable variadics here, it shouldn't be too hard to support this feature.

Linux kernel extensively uses this feature.

[jefftrull]

This looks interesting and useful for handling the Linux kernel, an important target for analysis. We should be sure to guard/enable it appropriately as a GNU extension.