linker: lld: cortex_m: Place .ARM.exidx sections when using lld

Currently, .ARM.exidx input sections are only handled when building with
ld. When building with lld (and depending on the orphan section handling
policy configured), a few issues can arise:

  1. lld may produce warnings about the orphan section
  2. lld may place the input .ARM.exidx sections in unexpected ways--it
     seems lld does place the .ARM.exidx input sections in the expected
     .ARM.exidx output section, but it places them at the end of the
     section (after '__exidx_end').

To resolve the possible warning and unexpected placement, explicitly
handle .ARM.exidx sections when lld is used.

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>