Linux buildbots should use GCC 9.x to build master

[steven-johnson]

Per https://gcc.gnu.org/gcc-9/changes.html and https://stackoverflow.com/a/49119902/2137996, GCC 9.x is the first version of GCC in which C++17 support isn't "experimental"; we should upgrade the buildbots to use GCC9 when building on master. (This is easy to do; I'm opening an issue to ensure we have buy-in.)

Currently, all the linuxbots (x86 + arm) use GCC 7.x for building. I propose that we continue to build the release/10.x and release/11.x using those versions (to maintain compatibility), but transition to GCC 9.x for Halide 12 (aka master).

(Note that AFAIK there are no such issues with MSVC or Clang.)