mcount: tidy up ANSIfication

libc/gmon's mcount was ANSIfied in r124180, with libkern following over
a decade later, in r325988, but some minor discrepancies remained.
Update libc/gmon's mexitcount to an ANSI C function definition, and use
(void) for libkern-only functions that take no arguments.

Reported by:	bde

lib/libc/gmon/mcount.c

@@ -279,8 +279,7 @@ MCOUNT
 
 #ifdef GUPROF
 void
-mexitcount(selfpc)
-	uintfptr_t selfpc;
+mexitcount(uintfptr_t selfpc)
 {
 	struct gmonparam *p;
 	uintfptr_t selfpcdiff;

sys/libkern/mcount.c

@@ -58,7 +58,8 @@ __FBSDID("$FreeBSD$");
  * both frompcindex and frompc.  Any reasonable, modern compiler will
  * perform this optimization.
  */
-_MCOUNT_DECL(uintfptr_t frompc, uintfptr_t selfpc)	/* _mcount; may be static, inline, etc */
+/* _mcount; may be static, inline, etc */
+_MCOUNT_DECL(uintfptr_t frompc, uintfptr_t selfpc)
 {
 #ifdef GUPROF
 	int delta;
@@ -291,7 +292,7 @@ mexitcount(uintfptr_t selfpc)
 #endif
 
 void
-empty_loop()
+empty_loop(void)
 {
 	int i;
 
@@ -300,13 +301,13 @@ empty_loop()
 }
 
 void
-nullfunc()
+nullfunc(void)
 {
 	__asm __volatile("");
 }
 
 void
-nullfunc_loop()
+nullfunc_loop(void)
 {
 	int i;