string: make memcpy(), memset(), memcmp() and memmove() visible for LTO

string: make memcpy(), memset(), memcmp() and memmove() visible for LTO

It seems that sometimes (happening on ARM64, for example with
turris_mox_defconfig) GCC, when linking with LTO, changes the symbol
names of some functions, for example lib/string.c's memcpy() function to
memcpy.isra.0.

This is a problem however when GCC for a code such as this:
	struct some_struct *info = get_some_struct();
	struct some struct tmpinfo;
	tmpinfo = *info;
emits a call to memcpy() by builtin behaviour, to copy *info to tmpinfo.

This then results in the following linking error:
  .../lz4.c:93: undefined reference to `memcpy'
  .../uuid.c:206: more undefined references to `memcpy' follow

GCC's documentation says this about -nodefaultlibs option:
  The compiler may generate calls to "memcmp", "memset", "memcpy" and
  "memmove".  These entries are usually resolved by entries in libc.
  These entry points should be supplied through some other mechanism
  when this option is specified.

Make these functions visible by using the __used macro to avoid this
error.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>