LoongArch: Select ARCH_HAS_FAST_MULTIPLIER

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>