test-link-abi: fall back to lowest observed glibc version on new architectures

test-link-abi: fall back to lowest observed glibc version on new architectures

On architectures where glibc support was introduced after our baseline
(e.g. loongarch64, added in glibc 2.36), every imported symbol is
tagged with a version newer than the baseline, so the baseline check
fails spuriously. Detect this by taking the minimum version observed
across all binaries: if it exceeds the baseline, treat that minimum
as the effective baseline instead.

Follow up for d9600a2a

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>