[ELF][ARM] Garbage collection support for .ARM.exidx sections

.ARM.exidx sections have a reverse dependency on the section they have
a SHF_LINK_ORDER dependency on. In other words a .ARM.exidx section is
live only if the executable section it describes is live. We implement
this with a reverse dependency field in InputSection.

Adding the dependency to InputSection is the simplest implementation
but it could be moved out to a separate map if it were found to decrease
performance for non ARM targets.

Differential revision: https://reviews.llvm.org/D25234

llvm-svn: 283734