libgcc, Darwin: Drop the legacy library build for macOS >= 15 [PR116809].

We have been building a legacy libgcc_s.1 DSO to support code that
was built with older compilers.

From macOS 15,  the unwinder no longer exports some of the symbols used
in that library which (a) cuases bootstrap fail and (b) means that the
legacy library is no longer useful.

No open branch of GCC emits references to this library - and any already
-built code that depends on the symbols would need rework anyway.

	PR target/116809

libgcc/ChangeLog:

	* config.host: Build legacy libgcc_s.1 on hosts before macOS 15.
	* config/i386/t-darwin: Remove reference to legacy libgcc_s.1
	* config/rs6000/t-darwin: Likewise.
	* config/t-darwin-libgccs1: New file.

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