[PATCH V2] gdb/testsuite: Add LoongArch case in my-syscalls.S

[Zewei Yang <yangzewei@loongson.cn>]

This patch adds syscall wrappers for LoongArch64 to gdb/testsuite/lib/my-syscalls.S.

- Implements the missing `__loongarch64` section.
- Uses `li.w` to load the syscall number into `$a7`.
- Triggers system calls via `syscall 0`.
- Returns to the caller using `ret`.

This change fixes build errors when running
`gdb.threads/step-over-thread-exit-while-stop-all-threads.exp`
on LoongArch64 targets caused by the absence of the LoongArch-specific
syscall wrappers in `my-syscalls.S`.

Tested on: loongarch64-unknown-linux-gnu
Result: expected testcases passed.

Signed-off-by: Zewei Yang <yangzewei@loongson.cn>
---
 gdb/testsuite/lib/my-syscalls.S | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gdb/testsuite/lib/my-syscalls.S b/gdb/testsuite/lib/my-syscalls.S
index c514b32d..3e2e397d 100644
--- a/gdb/testsuite/lib/my-syscalls.S
+++ b/gdb/testsuite/lib/my-syscalls.S
@@ -63,6 +63,18 @@ NAME:				;\
 NAME ## _syscall:		;\
 	svc #0
 
+#elif defined(__loongarch64)
+
+/* LoongArch 64-bit syscall wrapper */
+#define SYSCALL(NAME, NR)       \
+.global NAME                    ;\
+NAME:                           ;\
+        li.w 	$a7, NR		;\
+        /* syscall number */ \
+NAME ## _syscall:               ;\
+        syscall 0                       ;\
+        ret
+
 #else
 # error "Unsupported architecture"
 #endif
-- 
2.43.0