gdb: restore some checks of debug flags

gdb: restore some checks of debug flags

This partially reverts some parts of the commit:

  commit 17417fb0ec9842de1774e1e76f1f11c00cdafc47
  Date:   Sat Oct 31 09:01:25 2020 -0400

      gdb, gdbsupport: add debug_prefixed_printf, remove boilerplate functions

This commit removed 3 places where some debug flags were being
checked.  The result was that debug tracing was being printed
unconditionally.

This commit adds back the 3 flag checks.

gdb/ChangeLog:

	* infrun.h (infrun_debug_printf): Add check of debug_infrun flag.
	(debug_prefixed_printf): Add check of debug_displaced flag.
	* linux-nat.c (linux_nat_debug_printf): Add check of
	debug_linux_nat flag.