Add a set of co-proccesor constraint_setting (feature request)

[nathaniel-brough]

This is a similar suggestion to #13 but more generic and could include other secondary processors. This could include fpu's, gpgpu's, ai proccesors, simd extensions, dsp extensions, fpga's etc.

An example of this might look like;

coproccesor/BUILD

licenses(["notice"])
package(
    default_visibility = ["//visibility:public"],
)

constraint_setting(name = "fpu")

# Cortex-M4 optional fpu
constraint_value(
    name = "VFPv4_SP", 
    constraint_setting = ":fpu",
)

# Cortex-M7 optional fpu
constraint_value(
    name = "VFPv5_D16", 
    constraint_setting = ":fpu",
)

constraint_setting(name="dsp")

constraint_value(
    name = "cortex_m_dsp", 
    constraint_setting = ":dsp",
)

constraint_setting(name="ai")

constraint_value(
    name = "coral_edge_tpu", 
    constraint_setting = ":ai",
)

constraint_setting(name="gpgpu")

constraint_value(
    name = "amd-opencl", 
    constraint_setting = ":gpgpu",
)

constraint_value(
    name = "nvidia-cuda", 
    constraint_setting = ":gpgpu",
)

There is one caveat with using bazel platforms/constraint settings here and that is if you had a device that could support two constraint_values for a given constraint_setting. Say that you put a "AMD" and a "nvidia" gpgpu into a device you couldn't specify multiple values for the 'gpgpu' constraint_setting. This could be a common recurring problem, specifically with integrated+dedicated gpgpu devices.

[aiuto]

For the same reasons as #13, this is not something we would support.