mtcp_restart.c: Use -O2 for sake of inlining
- On ARM/gcc-4.8, this seems to avoid a bug in finding strings (ro-data)
    after copying code to high memory and jumping to it.


git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2579 ba6dc0ae-232f-0410-883f-9ebac64fc565