[Driver][SYCL] Enable adding of default device triple for AOT

[mdtoguchi]

When specifying an AOT enabling triple (spir64_gen, spir64_fpga,
spir64_x86_64) without also providing the default triple (spir64),
enable the default triple as well as the AOT enabling triple.

[AGindinson]

As far as I understand, the large part of the product code here is about retaining backwards compatibility for AOT users' build scripts, namely the ones relying on shortened versions of -Xsycl-target-* options.

If such a change is required for a particular use scenario, could it be better to hide this behavior under a separate option (-fsycl-force-spirv-image[=false|true] or something)? This would:

• allow to minimize the code (basically, just adding the generic triple inside the toolchain-building functions on the condition of the new option;
• push the beneficiaries of such "AOT + SPIR-V behavior" to refactor their build scripts and fix -Xsycl-target-* configuration issues along the way (and it would still be a relatively simple switch). Besides, such users may want to, at some point, also pass -Xsycl-target-* build/link options to the generic images' descriptors. As far as I understand the current approach, this would force them into using the triple explicitly, which would kind of devalue the automation work;
• additionally, the rest of the AOT users would be immune from unexpected changes to the driver behavior (I can't come up with a quick example, but some backwards compatibility issues could still fire for applications/libraries after the introduction of a second device binary). A 15-40% increase in the executable's size without a possibility to revert to old behavior could also cause headaches for those shipping large heterogeneous libraries.

[mdtoguchi]

closing - replaced with #4175