runtime: add support for --buildmode=c-shared on loong64

These c-shared related CLs are follow up of CLs 455016, 455017, 455018. Here we
follow the LoongArch ELF psABI v2 standard, which requires the support of the
PCALAU12I instruction.

Updates golang#53301
Updates golang#58784

Change-Id: I7f1ddbf3b2470d610f12069d147aa9b3a6a96f32
Reviewed-on: https://go-review.googlesource.com/c/go/+/425474
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/rt0_linux_loong64.s

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 #include "textflag.h"
+#include "cgo/abi_loong64.h"
 
 TEXT _rt0_loong64_linux(SB),NOSPLIT|NOFRAME,$0
 	// In a statically linked binary, the stack contains argc,
@@ -13,6 +14,56 @@ TEXT _rt0_loong64_linux(SB),NOSPLIT|NOFRAME,$0
 	ADDV	$8, R3, R5	// argv
 	JMP	main(SB)
 
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
+TEXT _rt0_loong64_linux_lib(SB),NOSPLIT,$168
+	// Preserve callee-save registers.
+	SAVE_R22_TO_R31(3*8)
+	SAVE_F24_TO_F31(13*8)
+
+	// Initialize g as nil in case of using g later e.g. sigaction in cgo_sigaction.go
+	MOVV	R0, g
+
+	MOVV	R4, _rt0_loong64_linux_lib_argc<>(SB)
+	MOVV	R5, _rt0_loong64_linux_lib_argv<>(SB)
+
+	// Synchronous initialization.
+	MOVV	$runtime·libpreinit(SB), R19
+	JAL	(R19)
+
+	// Create a new thread to do the runtime initialization and return.
+	MOVV	_cgo_sys_thread_create(SB), R19
+	BEQ	R19, nocgo
+	MOVV	$_rt0_loong64_linux_lib_go(SB), R4
+	MOVV	$0, R5
+	JAL	(R19)
+	JMP	restore
+
+nocgo:
+	MOVV	$0x800000, R4                     // stacksize = 8192KB
+	MOVV	$_rt0_loong64_linux_lib_go(SB), R5
+	MOVV	R4, 8(R3)
+	MOVV	R5, 16(R3)
+	MOVV	$runtime·newosproc0(SB), R19
+	JAL	(R19)
+
+restore:
+	// Restore callee-save registers.
+	RESTORE_R22_TO_R31(3*8)
+	RESTORE_F24_TO_F31(13*8)
+	RET
+
+TEXT _rt0_loong64_linux_lib_go(SB),NOSPLIT,$0
+	MOVV	_rt0_loong64_linux_lib_argc<>(SB), R4
+	MOVV	_rt0_loong64_linux_lib_argv<>(SB), R5
+	MOVV	$runtime·rt0_go(SB),R19
+	JMP	(R19)
+
+DATA _rt0_loong64_linux_lib_argc<>(SB)/8, $0
+GLOBL _rt0_loong64_linux_lib_argc<>(SB),NOPTR, $8
+DATA _rt0_loong64_linux_lib_argv<>(SB)/8, $0
+GLOBL _rt0_loong64_linux_lib_argv<>(SB),NOPTR, $8
+
 TEXT main(SB),NOSPLIT|NOFRAME,$0
 	// in external linking, glibc jumps to main with argc in R4
 	// and argv in R5