gdb/testsuite: Add LoongArch case in my-syscalls.S

When running the following command on LoongArch:

  git clone git://sourceware.org/git/binutils-gdb.git gdb
  mkdir -p build && cd build && ../gdb/configure && make -j"$(nproc)"
  make check-gdb TESTS="gdb.threads/step-over-thread-exit-while-stop-all-threads.exp"

there exists the following error:

  gdb/gdb/testsuite/lib/my-syscalls.S:67:3: error: #error "Unsupported architecture"

this is because there is no SYSCALL macro for LoongArch, just add it.
With this patch, the above test passes on LoongArch.

Signed-off-by: Zewei Yang <yangzewei@loongson.cn>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

gdb/testsuite/lib/my-syscalls.S

@@ -63,6 +63,17 @@ NAME:				;\
 NAME ## _syscall:		;\
 	svc #0
 
+#elif defined(__loongarch64)
+
+#define SYSCALL(NAME, NR)	\
+.global NAME			;\
+NAME:				;\
+        li.w	$a7, NR		;\
+	/* a0, a1 and a2 already contain the right arguments.  */ \
+NAME ## _syscall:		;\
+        syscall 0		;\
+        ret
+
 #else
 # error "Unsupported architecture"
 #endif