std.Target: allow omitting features

[alichraghi]

Some amdgcn gpu models (e.g. gfx1031) may or may not support XNACK and SRAMECC features, therefore LLVM has multiple settings for them (On, Off, Unsupported and Any). Since zig features are either on or off, there's no way to set them to Unsupported because std.Target has no concept of omitting features. making it impossible to compile for such gpu models because LLVM writes an error (with warning: label/prefix..).

Related: llvm/llvm-project@2291bd1
Zulip discussion: #compiler > dynamic amdgcn target features

[alexrp]

This'll probably require a complete rethink of how we process and store CPU features in std.Target.