fix: check .gopclntab before .data.rel.ro for PIE binaries

[jiridanek]

Summary

Go 1.26 changed the ELF layout for PIE builds: .gopclntab is now emitted as a
separate section instead of being embedded in .data.rel.ro (the Go <= 1.25
behavior). ReadTable unconditionally redirected to .data.rel.ro when it saw
-buildmode=pie, causing "could not find magic number" failures on any Go
1.26-built PIE binary.

This fix always checks .gopclntab first, falling back to .data.rel.ro only
if absent. This is backward-compatible since Go <= 1.25 PIE binaries don't have
.gopclntab.

Fixes #329

Evidence

Binary comparison of two s390x skopeo builds from RHEL 9 container images:

Binary	Go version	.gopclntab section	Magic in .data.rel.ro	check-payload result
skopeo 1.22.2 (appstream)	go1.26.1	Present (magic at offset 0)	Not present	FAIL
skopeo 1.18.1 (appstream-eus)	go1.25.8	Absent	Present	PASS

This will affect all architectures once more RPMs are rebuilt with Go 1.26.

Made with Cursor

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