`MOVK` must not be negative

[kper]

According to https://developer.arm.com/documentation/dui0802/a/A64-General-Instructions/MOVK, the imm range is between 16-bit unsigned immediate, in the range 0 to 65535..

However, the instructions in our specification print the immediate with the decimal function.

assembly MOVKWPos16 = ("movk", ' ', WSize(rd), ', #', decimal(imm16), ", LSL #16")