build: use macOS 11 SDK (Xcode 12.2)

This should be sufficient to support building for Apple ARM when
cross-compiling.

depends/hosts/darwin.mk

@@ -1,7 +1,7 @@
 OSX_MIN_VERSION=10.15
-OSX_SDK_VERSION=10.15.6
-XCODE_VERSION=12.1
-XCODE_BUILD_ID=12A7403
+OSX_SDK_VERSION=11.0
+XCODE_VERSION=12.2
+XCODE_BUILD_ID=12B45b
 LD64_VERSION=609
 
 OSX_SDK=$(SDK_PATH)/Xcode-$(XCODE_VERSION)-$(XCODE_BUILD_ID)-extracted-SDK-with-libcxx-headers