libc: move calloc() into its own compilation unit

move calloc() in its own compilation unit to avoid
code duplication. also, calloc() is used rarely in
plan9 programs.

sys/src/libc/port/calloc.c

@@ -0,0 +1,13 @@
+#include <u.h>
+#include <libc.h>
+
+void*
+calloc(ulong n, ulong s)
+{
+	void *v;
+	if(n > 1 && ((ulong)-1)/n < s)
+		return nil;
+	if(v = mallocz(n*s, 1))
+		setmalloctag(v, getcallerpc(&n));
+	return v;
+}

sys/src/libc/port/malloc.c

@@ -279,18 +279,6 @@ msize(void *v)
 	return poolmsize(mainmem, (ulong*)v-Npadlong)-Npadlong*sizeof(ulong);
 }
 
-void*
-calloc(ulong n, ulong s)
-{
-	void *v;
-
-	if(n > 1 && ((ulong)-1)/n < s)
-		return nil;
-	if(v = mallocz(n*s, 1))
-		setmalloctag(v, getcallerpc(&n));
-	return v;
-}
-
 void
 setmalloctag(void *v, uintptr pc)
 {

sys/src/libc/port/mkfile

@@ -12,6 +12,7 @@ CFILES=\
 	atof.c\
 	atol.c\
 	atoll.c\
+	calloc.c\
 	cistrcmp.c\
 	cistrncmp.c\
 	cistrstr.c\