checklinkname: rudimentary type-checking of linkname directives

This CL introduces a 'checklinkname' analyzer, which provides rudimentary
type-checking that verifies that function signatures on the local and remote
sides of //go:linkname directives match expected values.

If the Go standard library changes the definitions of any of these function,
checklinkname will flag the change as a finding, providing an error informing
the gVisor team to adapt to the upstream changes. This allows us to eliminate
the majority of gVisor's forward-looking negative build tags, as we can catch
mismatches in testing [1].

The remaining forward-looking negative build tags are covering shared struct
definitions, which I hope to add to checklinkname in a future CL.

[1] Of course, semantics/requirements can change without the signature
changing, so we still must be careful, but this covers the common case.

PiperOrigin-RevId: 387873847

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