linux-user: fix build on hosts not using guest base

linux-user: fix build on hosts not using guest base

Commit 68a1c816868b3e35a1da698af412b29e61b1948a broke qemu on hosts not
using guest base. It uses reserved_va unconditionally in mmap.c. To
avoid to many #ifdef #endif blocks, define RESERVED_VA as either
reserved_va or 0ul, and use it instead of reserved_va, similarly to what
has been done with guest_base/GUEST_BASE.