aarch64/sljit_machdep.h: Make this work in compat32 context.

Should fix clang build of compat32 eabi libsljit:

dependall ===> compat/arm/eabi/../../../lib/../external/bsd/sljit/lib
In file included from /home/source/ab/HEAD-llvm/src/sys/external/bsd/sljit/dist/sljit_src/sljitLir.c:1678:
/home/source/ab/HEAD-llvm/src/sys/external/bsd/sljit/dist/sljit_src/sljitNativeARM_64.c:142:54: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
        FAIL_IF(push_inst(compiler, MOVK | RD(dst) | (((imm >> 32) & 0xffff) << 5) | (2 << 21)));
                                                            ^  ~~

(riastradh)