parts of drreg are not well ported to AArch64: 32-bit write clears 64-bit; avoid SP

Noticed a couple of things in drreg that are not well ported to AArch64:

GPR liveness: the IF_X86_64 for 32-bit reg write zeroing top 32 should be IF_X64 to cover AArch64 too.

s/IF_NOT_ARM/IF_NOT_AARCHXX/ here:

                 * On AArch64 use of SP is very restricted.
                 */
                IF_NOT_ARM(|| reg == DR_REG_XSP))