pkg/cover: ensure that all PCs returned by kcov have matching callbacks

In the case some modules' addresses are off, certain kernel addresses
returned by kcov may not have corresponding coverage callbacks in the
.ko files. Keep an additional map in the backend to verify those
addresses and report an error if that is the case.

Because GCC < 14 may tail-call coverage callbacks, the described check
is not performed for binaries which mention GCC in their .comment
section.

Also adjust text expectations in pkg/cover/report_test.go, so that
non-GCC targets check for PCs matching the callbacks.

See https://github.com/google/syzkaller/issues/4447 for more details.

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