linux-user: Handle filesz < memsz for any PT_LOAD segment.

linux-user: Handle filesz < memsz for any PT_LOAD segment.

I caught padzero not properly initializing the .bss segment
on a statically linked Alpha program.  Rather than a minimal
patch, replace the gross code with a single mmap+memset.

Share more code between load_elf_interp and load_elf_binary.

Legally, an ELF program need not have just a single .bss;
and PT_LOAD segment can have memsz > filesz.

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