With help from chapuni on IRC, I think this is right now. =] The issue
is that the command is quoted differently from the arguments. The
command has '\' and the argument has '\\'. This is made unclear because
FileCheck escapes the single matched '\' when it prints the contents of
the variable, thus fooling me into thinking it had matched '\\' as
intended. The solution is to bind the gcc_install variable in the
argument list rather than out of the command. To do so we also have to
be a bit more careful so that we don't get stray other things into the
'.*' regex.

Also, because of the argument difference, '\\\\' is the correct
formulation before crtbegin, go back to that.