Change of toolchain registering API to simplify multiple execution platforms existing for toolchains.

[BrandonThomasJonesARM]

Following on from the PR that has been merged (#196)

We now have a toolchain registeration API that looks like the following:

bazel_dep(
    name = "hermetic_cc_toolchain",
    version = "",
)

toolchains = use_extension("@hermetic_cc_toolchain//toolchain:ext.bzl", "toolchains")
use_repo(toolchains, "zig_sdk", "zig_sdk-linux-amd64", "zig_sdk-linux-arm64", "zig_sdk-macos-amd64", "zig_sdk-macos-arm64", "zig_sdk-windows-amd64")

_COMMON_EXEC_PLATFORMS = [
    ("linux", "amd64"),
    ("linux", "arm64"),
    ("windows", "amd64"),
    ("macos", "arm64"),
    ("macos", "amd64"),
]

[register_toolchains(
    # if no `--platform` is specified, these toolchains will be used for
    # (linux,darwin,windows)x(amd64,arm64)
    "@zig_sdk-{}-{}//toolchain:linux_amd64_gnu.2.28".format(os, arch),
    "@zig_sdk-{}-{}//toolchain:linux_arm64_gnu.2.28".format(os, arch),
    "@zig_sdk-{}-{}//toolchain:windows_amd64".format(os, arch),
    "@zig_sdk-{}-{}//toolchain:windows_arm64".format(os, arch),
    # "@zig_sdk-{}-{}//toolchain:darwin_amd64".format(os, arch),
    # "@zig_sdk-{}-{}//toolchain:darwin_arm64".format(os, arch),

    # amd64 toolchains for libc-aware platforms:
    "@zig_sdk-{}-{}//libc_aware/toolchain:linux_amd64_gnu.2.28".format(os, arch),
    "@zig_sdk-{}-{}//libc_aware/toolchain:linux_amd64_gnu.2.31".format(os, arch),
    "@zig_sdk-{}-{}//libc_aware/toolchain:linux_amd64_musl".format(os, arch),
    # arm64 toolchains for libc-aware platforms:
    "@zig_sdk-{}-{}//libc_aware/toolchain:linux_arm64_gnu.2.28".format(os, arch),
    "@zig_sdk-{}-{}//libc_aware/toolchain:linux_arm64_musl".format(os, arch),
    # wasm/wasi toolchains
    "@zig_sdk-{}-{}//toolchain:wasip1_wasm".format(os, arch),

    # These toolchains are only registered locally.
    dev_dependency = True,
) for os, arch in _COMMON_EXEC_PLATFORMS]

Technically a user of these toolchains shouldn't need to be informed of the exec platforms available but should get them for "free". I think a better longterm API would be to have something along the lines of:

bazel_dep(
    name = "hermetic_cc_toolchain",
    version = "some-future-version",
)

toolchains = use_extension("@hermetic_cc_toolchain//toolchain:ext.bzl", "toolchains")
use_repo(toolchains, "zig_toolchains")

register_toolchains("@zig_toolchains/...")
#or 
register_toolchains(
    "@zig_toolchains//amd64-windows/...",  # This registers the `amd64-windows` target for all executor platforms known to `hermetic_cc_toolchain`
    "@zig_toolchains//arm64-windows/...",
    "@zig_toolchains//amd64-linux/...",
    "@zig_toolchains//arm64-linux/...",
    "@zig_toolchains//amd64-macos/...",
    "@zig_toolchains//arm64-macos/...",
)

Then, in future zig versions, if more platforms are supported, it does not require the user to list the extra execution platforms.

Since we know the names of each individual toolchain within a execution platforms toolchain repository as they're generated from def target_structs(): it should be possible to create the alias symlink tree needed to create this.

e.g.

#@zig_toolchains/amd64-linux/BUILD.bazel
#                     ^ this is the target platform

alias(
  name = "linux_amd64_gnu.2.28",
#               ^ This is the exec platform
  actual = "@zig_sdk-linux-amd64//toolchain:linux_amd64_gnu.2.28",
)
alias(
  name = "linux_arm64_gnu.2.28",
#               ^ This is the exec platform
  actual = "@zig_sdk-linux-arm64//toolchain:linux_amd64_gnu.2.28",
)

I am happy to implement this and submit another PR, but want to make sure you're happy with the design proposal first.