Fix: Don't assume that PROT_WRITE grants read permissions

The prot flag passed to mmap() is set to PROT_WRITE when the O_RDWR
access flags is used. This assumes that PROT_WRITE grants read
permissions on the mmap'ed region. While this is true on x86, this
causes a segmentation fault on SPARC and, presumably, other
architectures implementing strict access permissions.

CTF Writer needs read permissions since the unaligned integer writes
may use load instructions as the value is read back to perform the
required shifting/masking is performed in the _bt_bitfield_write_*
macros.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>