linux-user: Fixes for zero_bss

linux-user: Fixes for zero_bss

The previous change, 2d385be6152, assumed !PAGE_VALID meant that
the page would be unmapped by the elf image.  However, since we
reserved the entire image space via mmap, PAGE_VALID will always
be set.  Instead, assume PROT_NONE for the same condition.

Furthermore, assume bss is only ever present for writable segments,
and that there is no page overlap between PT_LOAD segments.
Instead of an assert, return false to indicate failure.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1854
Fixes: 2d385be6152 ("linux-user: Do not adjust zero_bss for host page size")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>