cmd/internal/obj/loong64: add the PCALAU12I instruction for reloc use

The LoongArch ELF psABI v2.00 revamped the relocation design, largely
moving to using the `pcalau12i + addi/ld/st` pair for PC-relative
addressing within +/- 32 bits. The "pcala" in `pcalau12i` stands for
"PC-aligned add"; the instruction's semantics happen to coincide with
arm64's `adrp`.

Add support for emitting this instruction as part of the relevant
addressing ops, for use with new reloc types later.

Updates #58784

Change-Id: Ic1747cd9745aad0d1abb9bd78400cd5ff5978bc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/455016
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Auto-Submit: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/internal/obj/loong64/asm.go

@@ -1847,6 +1847,8 @@ func (c *ctxt0) opir(a obj.As) uint32 {
 		return 0x0a << 25
 	case ALU32ID:
 		return 0x0b << 25
+	case APCALAU12I:
+		return 0x0d << 25
 	case APCADDU12I:
 		return 0x0e << 25
 	}