x86/boot: Provide KASAN compatible aliases for string routines

The KASAN subsystem wraps calls to memcpy(), memset() and memmove()
to sanitize the arguments before invoking the actual routines, which
have been renamed to __memcpy(), __memset() and __memmove(),
respectively. When CONFIG_KASAN is enabled for the kernel build but
KASAN code generation is disabled for the compilation unit (which is
needed for things like the EFI stub or the decompressor), the string
routines are just #define'd to their __ prefixed names so that they
are simply invoked directly.

This does however rely on those __ prefixed names to exist in the
symbol namespace, which is not currently the case for the x86
decompressor, which may lead to errors like

  drivers/firmware/efi/libstub/tpm.o: In function `efi_retrieve_tpm2_eventlog':
  tpm.c:(.text+0x2a8): undefined reference to `__memcpy'

So let's expose the __ prefixed symbols in the decompressor when
KASAN is enabled.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Garrett <matthewgarrett@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>