Make hwpmc work for userspace binaries again

hwpmc has been utterly broken for userspace binaries, and has been
labeling all samples from userspace binaries as dubious frames. The
issues are that:

-The check for ph.p_offset & (-ph.p_align) == 0 was mostly bogus. The
intent was to ignore all executable segments other than the first,
which when using BFD appeared in the first page, but with current LLD
a read-only data segment appears before the executable segment,
pushing the latter into the second page or later. This meant no
executable segment was ever found, and thus pi_vaddr remained
0. Instead of relying on BFD's layout, track whether we've seen an
executable segment explicitly with a local bool.

-Shared libraries were not parsing the segments to calculate pi_vaddr,
resulting in it always being 0. Again, when using BFD, the executable
segment started at the first page, and so pi_vaddr was genuinely
meant to be 0, but not with LLD's current layout. This meant that
pmcstat_image_link's offset calculation gave the base address of the
segment in memory, rather than the base address of the whole library
in memory, and so when adding that to pi_start/pi_end to get the
range of the executable sections in memory it double-counted the
offset of the first executable segment within the library. Thus we
need to do the exact same parsing for ET_DYN as we do for ET_EXEC,
which is simpler to write as special-casing ET_REL to not look for
segments. Note that, whilst PT_INTERP isn't needed for shared
libraries, it will be for PIEs, which pmcstat still fails to handle
due to not knowing the base address of the PIE; we get the base
address for libraries by MAP_IN events, and for rtld by virtue of the
process's entry address being rtld's, but have no equivalent for the
executable.

Fixes courtesy of jrtc27@.

Reviewed by: jrtc27, jhb (earlier version)
Differential Revision: https://reviews.freebsd.org/D33055
Sponsored by: Netflix