libmakepkg: Do not "strip" files when the format is not recognised

We use a multi step process during stripping to ensure permissions do
not get changed.  However, if the initial objcopy fails, the subsequent cat
results in a blank file.  Abandon early if objcopy fails.

Fixes FS#74486

Signed-off-by: Allan McRae <allan@archlinux.org>
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