linux-user: Honor elf alignment when placing images
Most binaries don't actually depend on more than page alignment,
but any binary can request it. Not honoring this was a bug.
This became obvious when gdb reported
Failed to read a valid object file image from memory
when examining some vdso which are marked as needing more
than page alignment.
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>