x86: slightly re-arrange 32-bit handling in dom0_construct_pv()

x86: slightly re-arrange 32-bit handling in dom0_construct_pv()

Add #ifdef-s (the 2nd one will be needed in particular, to guard the
uses of m2p_compat_vstart and HYPERVISOR_COMPAT_VIRT_START()) and fold
duplicate uses of elf_32bit().

Also adjust what gets logged: Avoid "compat32" when support isn't built
in, and don't assume ELF class <> ELFCLASS64 means ELFCLASS32.

While doing this, in code getting touched anyway:
- use ROUNDUP() instead of open-coding it,
- drop a stale (dead) BUG_ON(),
- replace panic() by printk() plus error return, for being consistent
  with other code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>