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>