LoongArch: Enable generic CPU vulnerabilites support

Currently, many architectures support generic CPU vulnerabilites, such
as x86, arm64 and riscv:

 commit 61dc0f55 ("x86/cpu: Implement CPU vulnerabilites sysfs functions")
 commit 61ae1321 ("arm64: enable generic CPU vulnerabilites support")
 commit 0e3f3649 ("riscv: Enable generic CPU vulnerabilites support")

All LoongArch CPUs (since Loongson-3A5000) implement a special mechanism
in the processor core to prevent "Meltdown" and "Spectre" attacks, so it
can enable generic CPU vulnerabilites support for LoongArch too.

Without this patch, there are no user interfaces of vulnerabilities to
check on LoongArch. The output of those files reflects the state of the
CPUs in the system, the output value "Not affected" means "CPU is not
affected by the vulnerability".

Before:

 # cat /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
 cat: /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow: No such file or directory
 # cat /sys/devices/system/cpu/vulnerabilities/spec_store_bypass
 cat: /sys/devices/system/cpu/vulnerabilities/spec_store_bypass: No such file or directory
 # cat /sys/devices/system/cpu/vulnerabilities/meltdown
 cat: /sys/devices/system/cpu/vulnerabilities/meltdown: No such file or directory

After:

 # cat /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
 Not affected
 # cat /sys/devices/system/cpu/vulnerabilities/spec_store_bypass
 Not affected
 # cat /sys/devices/system/cpu/vulnerabilities/meltdown
 Not affected

Link: https://www.loongson.cn/EN/news/show?id=633


Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>