runtime: Do not reserve huge amount of swap on 32 bit architectures.

runtime: Do not reserve huge amount of swap on 32 bit architectures.

The mmap() call which reserves the arena should have MAP_NORESERVE
flag as in typical cases this memory will never be (fully) needed.
This matters in environments which do not do Linux style memory
overcommit, such as OpenIndiana/OpenSolaris/Solaris.

The MAP_NORESERVE flag does not exist on all operating systems
(for example FreeBSD). Therefore we define it to zero value in
case it does not exist.

Fixes issue 21.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@196088 138bc75d-0d04-0410-961f-82ee72b054a4