Add fpu constraints

Older ARM cores can be configured without an FPU or without NEON, which causes general-purpose code to run much slower so LLVM assumes that these are always available. For most silicon out there this is true, but there are exceptions, some of which we care about so this constraint can select these non-standard core configs. This proposes the following addition:

```starlark
# //fpu/BUILD.bazel

constraint_setting(
name = "fpu",
default_constraint_value = ":neon",
)

constraint_value(
name = "neon",
constraint_setting = ":fpu",
)

constraint_value(
name = "vfpv4-d16",
constraint_setting = ":fpu",
)
```

Start with //fpu/BUILD.bazel, the file named in the issue. Add the proposed fpu constraint setting and its neon and vfpv4-d16 values, then verify that the BUILD file is accepted by the repository's Bazel checks. Done means the constraint can select these non-standard ARM core configurations.

Written by the indexing model from the issue text.