LoongArch: Simplify the invtlb wrappers

The invtlb instruction has been supported by upstream LoongArch
toolchains from day one, so ditch the raw opcode trickery and just use
plain inline asm for it.

While at it, also make the invtlb asm statements barriers, for proper
modeling of the side effects. The functions are also marked as
__always_inline instead of just "inline", because they cannot work at
all if not inlined: the op argument will not be compile-time const in
that case, thus failing to satisfy the "i" constraint.

The signature of the other more specific invtlb wrappers contain unused
arguments right now, but these are not removed right away in order for
the patch to be focused. In the meantime, assertions are added to ensure
no accidental misuse happens before the refactor. (The more specific
wrappers cannot re-use the generic invtlb wrapper, because the ISA
manual says $zero shall be used in case a particular op does not take
the respective argument: re-using the generic wrapper would mean losing
control over the register usage.)

Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>