Avoid doing arithmetic on void pointers.

Whatever compiler settings AOSP is using warns that this is a GNU extension.

Change-Id: Ife395d2b206b607b14c713cbb5a94d479816dad0
Reviewed-on: https://boringssl-review.googlesource.com/7604
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/cpu-arm-linux.c

diff --git a/crypto/cpu-arm-linux.c b/crypto/cpu-arm-linux.c
index 342fed2..ab4648c 100644
--- a/crypto/cpu-arm-linux.c
+++ b/crypto/cpu-arm-linux.c
@@ -64,12 +64,13 @@
 /* read_full reads exactly |len| bytes from |fd| to |out|. On error or end of
  * file, it returns zero. */
 static int read_full(int fd, void *out, size_t len) {
+  char *outp = out;
   while (len > 0) {
-    ssize_t ret = read_eintr(fd, out, len);
+    ssize_t ret = read_eintr(fd, outp, len);
     if (ret <= 0) {
       return 0;
     }
-    out += ret;
+    outp += ret;
     len -= ret;
   }
   return 1;