[jdk18] RFR: 8279702: [macosx] ignore xcodebuild warnings on M1

[Goetz Lindenmaier]

On Wed, 12 Jan 2022 09:42:07 GMT, Johannes Bechberger <duke at openjdk.java.net> wrote:

> Simple change to `make/autoconf/toolchain.m4` to fix the bug as described in its title and description.

Good point to write stderr to the output. This will simplify detecting future errors here.
LGTM

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Marked as reviewed by goetz (Reviewer).

PR: https://git.openjdk.java.net/jdk18/pull/95