Fix -Wempty-body warnings in glibc.

One group of warnings seen building glibc with -Wextra is -Wempty-body
warnings about an 'if' body (or in one case an 'else' body) that is
just a semicolon, "warning: suggest braces around empty body in an
'if' statement [-Wempty-body]" - I think the point of the warning
being to make it more visible whether an 'if' body is actually present
or not.

This patch fixes such warnings in glibc.  There's one place, with a
semicolon at the end of a comment, where this is clearly making the
presence of an 'else' body more visible.  The other cases involve
macro definitions expanding to nothing.  While there's no issue there
with visibility at the call sites, I think it's still cleaner to have
a macro that expands to something nonempty appropriate for the context
- so do {} while (0) if it's only intended to be usable as a
statement, or ((void) 0) where the macro definition is an alternative
to a call to a function returning void, so this patch makes those
changes.

Tested for x86_64.

	* catgets/gencat.c (normalize_line): Use braces around empty
	'else' body.
	* include/stap-probe.h [!USE_STAP_PROBE && !__ASSEMBLER__]
	(STAP_PROBE0): Use do {} while (0) for do-nothing definition.
	[!USE_STAP_PROBE && !__ASSEMBLER__] (STAP_PROBE1): Likewise.
	[!USE_STAP_PROBE && !__ASSEMBLER__] (STAP_PROBE2): Likewise.
	[!USE_STAP_PROBE && !__ASSEMBLER__] (STAP_PROBE3): Likewise.
	[!USE_STAP_PROBE && !__ASSEMBLER__] (STAP_PROBE4): Likewise.
	* libio/libio.h (_IO_funlockfile): Use ((void) 0) for do-nothing
	definition.

libio/libio.h

@@ -240,7 +240,7 @@ extern int _IO_ftrylockfile (FILE *) __THROW;
 
 #define _IO_peekc(_fp) _IO_peekc_unlocked (_fp)
 #define _IO_flockfile(_fp) /**/
-#define _IO_funlockfile(_fp) /**/
+#define _IO_funlockfile(_fp) ((void) 0)
 #define _IO_ftrylockfile(_fp) /**/
 #ifndef _IO_cleanup_region_start
 #define _IO_cleanup_region_start(_fct, _fp) /**/