The current imported libsanitizer code produces kernel panics for
Darwin 11 (macOS 10.7) and is unsupported for earlier versions already.

It is not clear if the current sources are even intended to be supported
on Darwin 11, so this patch causes the default to be build without
sanitizers for Darwin <= 11.

2020-03-01  Iain Sandoe  <iain@sandoe.co.uk>

	PR sanitizer/93731
	* configure.tgt (x86_64-*-darwin*, i?86-*-darwin*): Enable by
	default only for Darwin versions greater than 12 (macOS 10.8).