Revert "libc/memset: Implement function in assembler"

This reverts commit e7d344de01.
 This reverts the patch https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/5313 due to a timing issue with the merge.  The merge occurred at the same time as the additional comments and thusly were were not seen until the merge was done.  This reverts the change and additional patches from Alexei will follow to address the concerns expressed in the orignal patch.

Change-Id: Iae5f6403c93ac13ceeda29463883fcd4c437f2b7

lib/libc/memset.c

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <stddef.h>
+#include <string.h>
+
+void *memset(void *dst, int val, size_t count)
+{
+	char *ptr = dst;
+
+	while (count--)
+		*ptr++ = val;
+
+	return dst;
+}