move getDwarfExceptionSection from TAI to TLOF and rename it to

getLSDASection() to be more specific.  This makes it pretty obvious
that the ELF LSDA section is being specified wrong in PIC mode.  We're
probably getting a lot of startup-time relocations to a readonly page,
which is expensive and bad.

Someone who cares about ELF C++ should investigate this.

llvm-svn: 77847