ld: fix alignment of rodata section

We were not aligning the code size,
so read-only data, which follows in the same
segment, could be arbitrarily misaligned.

Fixes #2506.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5693055

src/cmd/ld/data.c

@@ -1023,6 +1023,11 @@ textaddress(void)
 		}
 		va += sym->size;
 	}
+	
+	// Align end of code so that rodata starts aligned.
+	// 128 bytes is likely overkill but definitely cheap.
+	va = rnd(va, 128);
+
 	sect->len = va - sect->vaddr;
 }