Change return value of _bfd_mmap_temporary

Change return value of _bfd_mmap_temporary

_bfd_mmap_temporary will return MAP_FAILED in some cases, but only one
caller was prepared for this.  This caused a couple of issues in gdb.

This patch chnages _bfd_mmap_temporary to always return NULL on
failure.  It also changes the logic to fall back to bfd_malloc if mmap
fails.

I took the opportunity to change some "(void *) -1" to MAP_FAILED
where appropriate.

Tested by "make check" in binutils, and also by applying a hack from
bug 33354 to gdb and testing something approximating the original
report.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32542
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33354