implement arm eabi mem* functions

these functions are part of the ARM EABI, meaning compilers may
generate references to them. known versions of gcc do not use them,
but llvm does. they are not provided by libgcc, and the de facto
standard seems to be that libc provides them.

arch/arm/src/__aeabi_memclr.c

+#include <string.h>
+#include "libc.h"
+
+void __aeabi_memclr(void *dest, size_t n)
+{
+	memset(dest, 0, n);
+}
+weak_alias(__aeabi_memclr, __aeabi_memclr4);
+weak_alias(__aeabi_memclr, __aeabi_memclr8);

arch/arm/src/__aeabi_memcpy.c

+#include <string.h>
+#include "libc.h"
+
+void __aeabi_memcpy(void *restrict dest, const void *restrict src, size_t n)
+{
+	memcpy(dest, src, n);
+}
+weak_alias(__aeabi_memcpy, __aeabi_memcpy4);
+weak_alias(__aeabi_memcpy, __aeabi_memcpy8);

arch/arm/src/__aeabi_memmove.c

+#include <string.h>
+#include "libc.h"
+
+void __aeabi_memmove(void *dest, const void *src, size_t n)
+{
+	memmove(dest, src, n);
+}
+weak_alias(__aeabi_memmove, __aeabi_memmove4);
+weak_alias(__aeabi_memmove, __aeabi_memmove8);

arch/arm/src/__aeabi_memset.c

+#include <string.h>
+#include "libc.h"
+
+void __aeabi_memset(void *dest, size_t n, int c)
+{
+	memset(dest, c, n);
+}
+weak_alias(__aeabi_memset, __aeabi_memset4);
+weak_alias(__aeabi_memset, __aeabi_memset8);