Fully precise gc handling of __start and __stop symbols.

This puts us at parity with bfd, which could already gc this case.

I noticed the sections not being gced when linking a modified freebsd
kernel. A section that was not gced and not mentioned in the linker
script would end up breaking the expected layout. Since fixing the gc
is relatively simple and an improvement, that seems better than trying
to hack the orphan placement code.

There are 173 input section in the entire link whose names are valid C
identifiers, so this is probably not too performance critical.

llvm-svn: 297049