For libgcc, we have (so far) supported building a DSO that supports
earlier versions of the OS than the target. From macOS 11, there are
APIs that do not exist on earlier OS versions, so limit the libgcc
range to macOS11..current.
libgcc/ChangeLog:
* config.host: From macOS 11, limit earliest macOS support
to macOS 11.
* config/t-darwin-min-11: New file.
Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>