Darwin: Check as for .build_version support and use it if available.

Darwin: Check as for .build_version support and use it if available.

This adds support for the minimum OS version data in assembler files.
At present, we have no mechanism to detect the SDK version in use, and
so that is omitted from build_versions.

We follow the implementation in clang, '.build_version' is only emitted
(where supported) for target macOS versions >= 10.14.  For earlier macOS
we fall back to using a '.macosx_version_min' directive.  This latter is
also emitted when the assembler supports it, but not build_version.

gcc/ChangeLog:

	* config.in: Regenerate.
	* config/darwin.cc (darwin_file_start): Add assembler directives
	for the target OS version, where these are supported by the
	assembler.
	(darwin_override_options): Check for building >= macOS 10.14.
	* configure: Regenerate.
	* configure.ac: Check for assembler support of .build_version
	directives.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>