introduce bb_putchar(). saves ~1800 on uclibc (less on glibc).

diff --git a/coreutils/cal.c b/coreutils/cal.c
index 35a5631..3116e1e 100644
--- a/coreutils/cal.c
+++ b/coreutils/cal.c
@@ -167,7 +167,7 @@
 			if (!julian) {
 				printf("%*s%s", HEAD_SEP, "", day_headings);
 			}
-			putchar('\n');
+			bb_putchar('\n');
 			for (row = 0; row < (6*7); row += 7) {
 				for (which_cal = 0; which_cal < 3-julian; which_cal++) {
 					dp = days[month + which_cal] + row;