Consider adding `toolchains` parameter to `configure_make_variant`

I'm trying to use rules_foreign_cc to build qemu. And at the moment, I'd like to use the preinstalled `pkgconfig` and preinstalled `meson` toolchains.

Unfortunately, it seems like `configure_make_variant` only allows for me to pass in a single toolchain, so I can't do something like:

```
configure_make_variant(
name = "qemu",
lib_source = "@qemu_dist//:all_srcs",
toolchains = ["@rules_foreign_cc//toolchains:preinstalled_meson_toolchain",
"@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain"]
)
```

Alternatively, if there was a way to compose toolchains, the existing `toolchain` parameter might work:

```
configure_make_variant(
name = "qemu",
lib_source = "@qemu_dist//:all_srcs",
toolchain = compose_toolchains(["@rules_foreign_cc//toolchains:preinstalled_meson_toolchain",
"@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain"])
)
```

Start at the implementation and documentation for configure_make_variant and trace how its existing toolchain parameter is handled. Investigate whether multiple toolchains can be accepted or composed, using the preinstalled_meson_toolchain and preinstalled_pkgconfig_toolchain examples as the target case. Done means a configure_make_variant invocation can use both preinstalled toolchains.

Written by the indexing model from the issue text.