configure: fix detection for x32 linux-user

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>