Use -target instead of -arch in compiling commands in Apple crosstool

[thii]

This allows using this crosstool file for cross-compiling from non-Apple
host platforms. On macOS, -arch <arch> and
-m<platform>-version-min=<version_min> will be expanded to -triple <arch>-apple-<platform><version_min> in the frontend invocation, but on
a Linux host, without -target, the expanded triple flags would be
-triple <arch>-pc-linux-gnu.

Fixes #11986

[keith]

If we're passing target we can probably drop the arch arg? Also see #13448 which should make this necessary in fewer places

[thii]

I'll rebase this after your PR is merged.

I've actually never seen the target being rewritten to aarch64-apple-darwin as mentioned in the linked commit, maybe it's only with older clang? If there's no problem I can drop the -arch argument.

[keith]

hrm i wonder if that has something to do with the host OS? so maybe that's only a problem with clang on non macOS hosts?

[thii]

Couldn't reproduce myself. Rebased and removed -arch.

Also it seems this will fix #11986.

[keith]

looks like the failure is related

[thii]

There's still some places in this file that can be simplified. I did a cleanup of it a while ago while implementing another toolchain, and the target system name can already be derived from cpu like:

def _target_system_name(cpu):
    platform, _, cpu = cpu.partition("_")
    if platform == "darwin":
        platform = "macosx"
    return "{}-apple-{}".format(cpu, platform)

https://github.com/apple-cross-toolchain/rules_applecross/blob/e59e4a02aa41b918b919d3711bb9c167b4474961/toolchain/cc_toolchain_config.bzl.tpl#L31-L35

[keith]

doing some of that in this #13449

[keith]

I think this is good to go, wdyt?

[thii]

I think we should land this. I would prefer the default crosstool to just work in most environments as much as it can.

Rebased and updated the target triple to also minimum OS version and target environment. With this, the version_min feature should be no longer needed, but we can clean it up in a follow-up.

[keith]

will this usage

bazel/tools/osx/crosstool/cc_toolchain_config.bzl

Line 3072 in ac48e65

target_system_name = target_system_name,

be happy with that change? not sure how it gets consumed later.

[thii]

It's what returned by cc_toolchain.target_gnu_system_name.

[thii]

Superseded by #14898.