rts: remove -Wl,-U,___darwin_check_fd_set_overflow hack

This patch bumps macOS minimum SDK version to 11.0 for x86_64-darwin to align it with aarch64-darwin. This allows us to get rid of the horrible -Wl,-U,___darwin_check_fd_set_overflow hack, which is causing linker warnings and testsuite failures on macOS 15. Fixes #25504 (closed).

I have run validate jobs for darwin on my macOS 15 runner, the annoying linker warnings are gone and the testsuite passes: https://gitlab.haskell.org/haskell-wasm/ghc/-/pipelines/103669

This should also allow us to upgrade existing macOS runners to 15 or add new macOS 15 runners in the future. And nothing needs to be redeployed for existing macOS runners: I've manually checked the instance runners through gitlab dashboard, and every single runner with x86_64-darwin-m1 tag also has aarch64-darwin tag which means they are already used to run aarch64-darwin jobs with minimum 11.0 SDK.