arm: fix compile warnings when semihosting is enabled on ARMv7M target.

This patch fixes compile warnings like this:

warning: format '%lu' expects argument of type 'long unsigned int',
         but argument 5 has type 'size_t'

In C99 standard you can use %zu modifier to print size_t values.

Signed-off-by: Vadzim Dambrouski <pftbest@gmail.com>

arch/arm/lib/semihosting.c

@@ -92,7 +92,7 @@ static long smh_read(long fd, void *memp, size_t len)
 		size_t len;
 	} read;
 
-	debug("%s: fd %ld, memp %p, len %lu\n", __func__, fd, memp, len);
+	debug("%s: fd %ld, memp %p, len %zu\n", __func__, fd, memp, len);
 
 	read.fd = fd;
 	read.memp = memp;
@@ -106,7 +106,7 @@ static long smh_read(long fd, void *memp, size_t len)
 		 * hard to maintain partial read loops and such, just fail
 		 * with an error message.
 		 */
-		printf("%s: ERROR ret %ld, fd %ld, len %lu memp %p\n",
+		printf("%s: ERROR ret %ld, fd %ld, len %zu memp %p\n",
 		       __func__, ret, fd, len, memp);
 		return -1;
 	}