gdb: move debug_prefixed_vprintf here

gdb: move debug_prefixed_vprintf here

The following patch needs to output debug prints from gdbsupport code.
Move debug_prefixed_vprintf so that it is possible to use it from
gdbsupport.

gdb/ChangeLog:

	* debug.c (debug_prefixed_vprintf): Move to gdbsupport.
	* debug.h: Remove.
	* infrun.c: Include gdbsupport/common-debug.h.
	* linux-nat.c: Likewise.

gdbsupport/ChangeLog:

	* common-debug.cc (debug_prefixed_vprintf): Move here.
	* common-debug.h (debug_prefixed_vprintf): Move here.

Change-Id: I5170065fc10a7a49c0f1bba67c691decb2cf3bcb