linux-user: Use zero_bss for PT_LOAD with no file contents too

If p_filesz == 0, then vaddr_ef == vaddr.  We can reuse the
code in zero_bss rather than incompletely duplicating it in
load_elf_image.

Tested-by: Helge Deller <deller@gmx.de>
Reviewed-by: Helge Deller <deller@gmx.de>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>