Add private mmap/munmap wrappers

Unlike Linux, Android (bionic) has separate functions - mmap & mmap64.
Add a local wrapper (inspired by gallium) that will be used throughout
libdrm to combat this bionic feature.

v2:
 - Drop the undefined unlikely macro.
 - Use macro for munmap under Android.
 - Wrap long lines.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com> (v1)