First step in fixing MC. Make it clear that we are avoiding a bug in the

First step in fixing MC. Make it clear that we are avoiding a bug in the
darwin9 linker, what is needed to avoid it and where to get more information.

Also make the workaround simpler. Just the regular end_sequence we normally
create is more than 4 bytes.

Tested by building cctools and ld64 from darwin9 on a darwin10 system and using
those. I checked that I was able to reproduce the bootstrap failure when
the the workaround was disabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120854 91177308-0d34-0410-b5e6-96231b3b80d8