Canonicalize output section name for .ARM.exidx sections

Canonicalize output section name for .ARM.exidx sections

The ARM Exception handling ABI requires that all ARM exception index
table sections have a prefix of .ARM.exidx and are combined into a
single contiguous block either in their own output section or as part
of another output section.

In general clang will output a single .ARM.exidx section per object,
but will use .ARM.exidx.<section name> when -ffunction-sections is used.

This change canonicalizes the names of sections with the .ARM.exidx
prefix to just .ARM.exidx, which ensures that there is only a single
output section.

Differential Revision: https://reviews.llvm.org/D23775

llvm-svn: 279617