bfd / PE/COFF: avoid setting SEC_ALLOC when finding IMAGE_SCN_MEM_DISCARDABLE

bfd / PE/COFF: avoid setting SEC_ALLOC when finding IMAGE_SCN_MEM_DISCARDABLE

Before teaching _bfd_XXi_swap_scnhdr_out() to respect SEC_ALLOC when
considering whether to force IMAGE_SCN_MEM_DISCARDABLE for a section,
make sure we won't "invert" present wrong behavior when transforming an
incoming object's section attributes to an outgoing one's (objcopy or
"ld -r"): Right now, IMAGE_SCN_MEM_DISCARDABLE would appear out of the
blue for certain sections. We don't want the opposite, though, i.e. we
want avoid silently dropping IMAGE_SCN_MEM_DISCARDABLE when it's present
for an incoming object's section.