linux-user: Fix fault address truncation AArch64

On AArch64 the si_addr field of siginfo_t is truncated to 32 bits
because the fault address passes through an uint32_t variable.

Follow Peters suggestion and drop the uint32_t variable
since its only used once in the Aarch64 loop.

Reported-by: Amanieu d'Antras <amanieu@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
