readelf: Add some support for locviews.

readelf: Add some support for locviews.

This adds minimal support for locviews as output by GCC8.
It changes readelf to keep track of loclistptrs from DW_AT_GNU_locviews
and prints the locview pairs for those. Since there is no terminator
we have to keep track of where the next loclist entry starts.
The --debug-dump=loc output looks as follows:

 CU [   714] base: +0x0000000000003020 <elf_hash>
 [    b4] view pair 1, 2
 [    b6] range 4, 7f
          +0x0000000000003024 <elf_hash+0x4>..
          +0x000000000000309e <elf_hash+0x7e>
           [ 0] reg5
 [    d9] view pair 3, 1
          view pair 1, 2
 [    dd] range 4, 4c
          +0x0000000000003024 <elf_hash+0x4>..
          +0x000000000000306b <elf_hash+0x4b>
           [ 0] reg5
          range 4c, 7f
          +0x000000000000306c <elf_hash+0x4c>..
          +0x000000000000309e <elf_hash+0x7e>
           [ 0] reg2

Note that in the above output the view pairs correspond to the ranges
immediately following in the loc list. This is how GCC8 currently
outputs the locview pairs and ranges, but this is not guaranteed and
you'll need to look at the location and GNU_locviews attributes of the
DIE to know which really match up together. We might want to adjust the
output to make this more clear.

This does not yet add an locview accessor to libdw. It just recognizes
the DW_AT_GNU_locviews attribute as a loclistptr when encoded as a
sec_offset form.

Signed-off-by: Mark Wielaard <mark@klomp.org>