linux-user: Fix trivial build error on loongarch64 hosts

linux-user: Fix trivial build error on loongarch64 hosts

When building using GCC 8.3.0 on loongarch64 (Loongnix) we get:

  In file included from ../linux-user/signal.c:33:
  ../linux-user/host/loongarch64/host-signal.h: In function ‘host_signal_write’:
  ../linux-user/host/loongarch64/host-signal.h:57:9: error: a label can only be part of a statement and a declaration is not a statement
         uint32_t sel = (insn >> 15) & 0b11111111111;
         ^~~~~~~~

We don't use the 'sel' variable more than once, so drop it.

Meson output for the record:

  Host machine cpu family: loongarch64
  Host machine cpu: loongarch64
  C compiler for the host machine: cc (gcc 8.3.0 "cc (Loongnix 8.3.0-6.lnd.vec.27) 8.3.0")
  C linker for the host machine: cc ld.bfd 2.31.1-system

Fixes: ad812c3bd65 ("linux-user: Implement CPU-specific signal handler for loongarch64 hosts")
Reported-by: Song Gao <gaosong@loongson.cn>
Suggested-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220104215027.2180972-1-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>