ARM64: Fix mterp switch table pointer calculation.

Do not mix 32-bit and 64-bit registers with
    add     x0, xPC, w0, lsl #1
that ends up compiled as
    add     x0, xPC, w0, uxtx #1
instead of the required sxtx. Just sing-extend the offset
correctly in previous instructions.

Test: Additional test in 501-regression-packed-switch.
Change-Id: I9867dc1180743e98f9707a312241d2f5b726ca8c