Move the GCC installation detection helper a member of the Linux

toolchain instead of merely using it in the constructor. This will allow
us to query it when building include paths as well as the file search
paths built in the constructor. I've lifted as little of it as I could
into the header file.

Eventually this will likely sink down into some of the Generic
toolchains and be used on more platforms, but I'm starting on Linux so
I can work out all the APIs needed there, where it is easiest to test
and we have the most pressing need.

llvm-svn: 143838