configure: fix detection for x32 linux-user

x32 uses the same signal handling fragments as x86_64, since host_arch
is set to x86_64 when Meson runs.  Remove the unnecessary forwarder and
set the host_arch variable properly in configure.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Message-ID: <20230808120303.585509-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -472,6 +472,8 @@ fi
 case "$cpu" in
   riscv*)
     host_arch=riscv ;;
+  x32)
+    host_arch=x86_64 ;;
   *)
     host_arch="$cpu" ;;
 esac

linux-user/include/host/x32/host-signal.h

@@ -1 +0,0 @@
-#include "../x86_64/host-signal.h"