[Arm64] Set Instruction set flags

[sdmaclea]

@CarolEidt @RussKeldorph @dotnet/jit-contrib @dotnet/arm64-contrib PTAL

[sdmaclea]

test Tizen armel Cross Checked Innerloop Build and Test

[CarolEidt]

This sequence is quite confusing. Wouldn't it be clearer just to start with InstructionSet_Base as the default?
e.g.:

// There is no JitFlag to specify the base instruction set; we start with that as the default.
opts.SetSupportedISA(InstructionSet_Base);
define HARDWARE_INTRINSIC_CLASS(flag, isa)                                                         \
    if (jitFlags.IsSet(JitFlags::flag))                                                            \
        opts.setSupportedISA(InstructionSet_##isa);
#include "hwintrinsiclistArm64.h"

[sdmaclea]

👍 Changing the order of lines would make it clearer.

@CarolEidt Are you also objecting to use of HARDWARE_INTRINSIC_CLASS_WITH_FLAG to remove InstructionSet_Base from the include list.

[CarolEidt]

OK, I think I get it now - you can't omit InstructionSet_Base from hwintrinsiclistarm64.h because it's included elsewhere where it will be required. So, you need a way to handle that without the flag. I can't think of a better way to do this off the top of my head, but I definitely think that setting InstructionSet_Base first would help in understanding, and then perhaps if you changed HARDWARE_INTRINSIC_CLASS_WITH_FLAG to HARDWARE_INTRINSIC_CLASS_WITH_FLAG_ONLY it would make it clear that it's a boolean control that ignores the one without a flag.

[sdmaclea]

I removed the HARDWARE_INTRINSIC_CLASS_WITH_FLAG and always handle base manually.

It is simpler and easier to understand.

[CarolEidt]

LGTM - thanks for making it clearer!