gdb/solib-rocm: avoid expensive gdbarch_from_bfd call in rocm_solib_relocate_section_...

gdb/solib-rocm: avoid expensive gdbarch_from_bfd call in rocm_solib_relocate_section_addresses

Loading a library containing a lot (> 100k) sections proved very slow
with whenever the support for ROCm was built into gdb.  The culprit is
the gdbarch_from_bfd call in rocm_solib_relocate_section_addresses:

    if (!is_amdgpu_arch (gdbarch_from_bfd (so.abfd.get ())))

This function gets called for every section, and gdbarch_from_bfd is
somewhat slow.  It turns out that we can skip the gdbarch_from_bfd call,
since all is_amdgpu_arch needs is the bfd_architecture value, which we
can directly extract from the `bfd *`, without going through the
gdbarch.

Add an overload of is_amdgpu_arch that takes a `bfd *`, and use it in
rocm_solib_relocate_section_addresses.

Update a call site in rocm_solib_bfd_open to use the new overload as
well.  That call site is not as much in a hot path, but there is no
point in paying the extra cost of looking up the gdbarch there.  I
removed the other assert that checked that gdbarch_from_bfd returned a
non-nullptr value.  If that was the case, something would be very wrong
with ROCgdb, and the problem would manifest very soon after anyway.

Change-Id: I55e9e68af59903b1b9727ff57388f9469d0e0002
Approved-by: Lancelot Six <lancelot.six@amd.com> (AMDGPU)