kernel_ldr: use unoptimized memcpy before mmu bringup (closes #1102)

Before the MMU is up, all reads/writes must be aligned; the optimized
memcpy implementation does not guarantee all reads/writes it performs
are aligned.

This commit splits the libc impl to be separate for kernel/kernel_ldr,
and so now only kernel will use the optimized impl. This is safe,
as the MMU is brought up before kernel begins executing.

mesosphere/kernel/source/libc/arch/arm64/asmdefs.h

@@ -0,0 +1,31 @@
+/*
+ * Macros for asm code.
+ *
+ * Copyright (c) 2019, Arm Limited.
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef _ASMDEFS_H
+#define _ASMDEFS_H
+
+#define ENTRY_ALIGN(name, alignment)    \
+  .global name;                         \
+  .type name,%function;                 \
+  .align alignment;                     \
+  name:                                 \
+  .cfi_startproc;
+
+#define ENTRY(name)    ENTRY_ALIGN(name, 6)
+
+#define ENTRY_ALIAS(name)   \
+  .global name;             \
+  .type name,%function;     \
+  name:
+
+#define END(name)       \
+  .cfi_endproc;         \
+  .size name, .-name;
+
+#define L(l) .L ## l
+
+#endif