Set OS and compiler constraints

[auscompgeek]

This sets rules_cc's compiler_flag to gcc for the cross-compiler platforms, to allow for easily selecting gcc targets (e.g. to specify compiler flags) downstream.

Example usage:

cc_library(
    # ...
    copts = select({
        "@bazel_tools//tools/cpp:msvc": [
            "/wd4005",
            "/wd4018",
            "/wd4244",
            "/wd4267",
            "/wd4996",
        ],
        "@bazel_tools//tools/cpp:gcc": [
            "-Wno-format-nonliteral",
            "-Wno-maybe-uninitialized",
            "-Wno-sign-compare",
            "-Wno-type-limits",
        ],
        "//conditions:default": [],
    }),
)

Also add the OS and compiler constraints to the cross-compilers, since that was also simple to do, to doubly-ensure nobody's trying to request contradictory compilers. (Adding the CPU constraint was too much for my 2am brain.)

[pjreiniger]

This seems to make sense to me, but @AustinSchuh is much more familiar with the nitty gritty of toolchains so I'd rather have him approve

[AustinSchuh]

That's reasonable. I still feel like there isn't a great consensus in the bazel community for the canonical way to select gcc vs clang, but this is as reasonable as any.

[auscompgeek]

Yeah, I figure since both the toolchain and platform definitions are being controlled here, it makes sense to make the constraint that already exists selectable.