asm-generic: Allow arch specific redefinition of read_word_at_a_time()

read_word_at_a_time() is used by strscpy() to copy the source buffer
into the destination one a word at a time.

Introduce __HAVE_ARCH_READ_WORD_AT_A_TIME macro to allow arch specific
redefinition of read_word_at_a_time().

This will be used on aarch64 with Memory Tagging Extension enabled and
enabled with a future patch.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>