arm64: msac: Explicitly use the ldur instruction
The ldr instruction can take an immediate offset which is a multiple of the loaded element size. If the ldr instruction is given an immediate offset which isn't a multiple of the element size, most assemblers implicitly generate a "ldur" instruction instead.
Older versions of MS armasm64.exe don't do this, but instead error out with "error A2518: operand 2: Memory offset must be aligned". (Current versions don't do this but correctly generate "ldur" implicitly.)
Switch this instruction to an explicit "ldur", like we do elsewhere, to fix building with these older tools.