bfd/PE: respect SEC_ALLOC when deciding whether to force IMAGE_SCN_MEM_DISCARDABLE

bfd/PE: respect SEC_ALLOC when deciding whether to force IMAGE_SCN_MEM_DISCARDABLE

While in the common case sections like .reloc don't need to be accessed
by a binary (that's the job of the loader), there are rare cases where
the situation is different. Unconditionally forcing the discardable flag
is therefore unhelpful. Avoid doing so when SEC_ALLOC is set, which
would in particular be the case when .reloc isn't (solely) linker-
generated.

While adjusting the comment, drop the wrong (stale?) following sentence:
.rsrc, as per its table entry, doesn't need to be writable. And
commenting on it wanting to be readable would then need to be repeated
for all other sections as well.