Add patch for GCC bug 58800.
Merge branch 'gcc-fix' of git://github.com/ambrop72/nixpkgs

Description from ambrop72:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58800

I have not been able to test this due to nix wanting to rebuild lots of
things before building gcc.
Patch was created with: svn diff -r 203872:203873
http://gcc.gnu.org/svn/gcc/branches

This should fix a crash in OpenSCAD: openscad/openscad#514