Arm64: further speed-up to hweight{32,64}()

According to Linux commit e75bef2a4f ("arm64: Select
ARCH_HAS_FAST_MULTIPLIER") this is a further improvement over the
variant using only bitwise operations on at least some hardware, and no
worse on other.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/arch/arm/Kconfig

@@ -12,6 +12,7 @@ config ARM_32
 config ARM_64
 	def_bool y
 	depends on 64BIT
+	select HAS_FAST_MULTIPLY
 
 config ARM
 	def_bool y