linux-user: Improve consistency checking in elf headers.

Validate more fields of the elf header.  Extract those checks
into two common functions to be used in both load_elf_interp
and load_elf_binary.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
