Toolchain rules don't consistently use `attrs.arg()`

[Ralith]

Compare:

buck2/prelude/toolchains/cxx.bzl

Line 232 in 0011c3f

"link_flags": attrs.list(attrs.string(), default = []),

buck2/prelude/toolchains/rust.bzl

Line 71 in 0011c3f

"rustc_flags": attrs.list(attrs.string(), default = []),

buck2/prelude/toolchains/python.bzl

Line 86 in 0011c3f

"linker_flags": attrs.default_only(attrs.list(attrs.arg(), default = [])),

Encountered this while trying to work around facebookincubator/reindeer#62 by referencing a filegroup's location in a toolchain's linker args. It's also weird that system_cxx_toolchain uses link_flags where linker_flags is common elsewhere. Copy-pasting the toolchain definitions and adjusting the attr types seems to work fine.

[Ralith]

This has now been addressed in system_cxx_toolchain and, shortly, in system_rust_toolchain. That's all the cases I'm personally aware of, so I'm calling this done 🎉