qemu-user: zero_bss should not assume PT_LOAD & PROT_WRITE

Goal

Support loading ELF segments with PT_LOAD & ~PROT_WRITE.

Technical details

Since e6e66b03, zero_bss has been assuming that PT_LOAD & PROT_WRITE. However it is possible to have an ELF file with PT_LOAD + (PROT_READ | PROT_EXEC) and to let Linux kernel load it.