Template variables for cxx_genrule do not work with system_cxx_toolchain

[thoughtpolice]

The documentation for cxx_genrule lists a bunch of macros that are available in the context of the command such as $(cxx) or $(cxxflags). These unkeyed TemplatePlaceholderInfo() variables are expected to come from toolchains//:cxx which is a dependency of cxx_genrule. These macros are very useful, obviously, for commands like bindgen that need access to the compiler flags.

However, it doesn't work OOTB with system_cxx_toolchain, because system_cxx_toolchain doesn't call cxx_toolchain_infos which sets up the appropriate TemplatePlaceholderInfo(). After some spelunking I found cxx_toolchain, a very handy rule, that does do that, and is a bit more appropriate for my use case where I download a toolchain and then point something to it.

Currently on Windows I use system_cxx_toolchain with a hack to avoid these template variables in that case, and though I'll probably migrate to cxx_toolchain on all platforms including Windows to mitigate this and make things more consistent, this should still probably be fixed OOTB for users.