LoongArch: Select ARCH_HAS_FAST_MULTIPLIER

LA464 and LA664 can do 32-bit/64-bit integer multiplication with a
latency of 4 cycles and a throughput of 2 ops per cycle.  It is
comparable to the mainstream x86 and arm64 cores, so we can select
ARCH_HAS_FAST_MULTIPLIER like them.

It speeds up __sw_hweight32() in lib/hweight.c for about 14% on LA464
and 11% on LA664, while __sw_hweight64() for about 30% on LA464 and 33%
on LA664.
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -16,6 +16,7 @@ config LOONGARCH
 	select ARCH_HAS_ACPI_TABLE_UPGRADE	if ACPI
 	select ARCH_HAS_CPU_FINALIZE_INIT
 	select ARCH_HAS_CURRENT_STACK_POINTER
+	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS