macos: Bump the minimum deployment target to macOS 10.12

Since https://github.com/llvm/llvm-project/pull/117252 and
https://github.com/llvm/llvm-project/commit/186fac33d08b34be494caa58fe63972f69c6d6ab,
part of the latest git main branch, LLDB uses std::shared_mutex.

On macOS, std::shared_mutex is only available since macOS 10.12.

This change raises the deployment target from 10.9 to 10.12.

build-lldb-mi.sh

@@ -111,7 +111,7 @@ fi
 
 if [ -n "$MACOS_REDIST" ]; then
     : ${MACOS_REDIST_ARCHS:=arm64 x86_64}
-    : ${MACOS_REDIST_VERSION:=10.9}
+    : ${MACOS_REDIST_VERSION:=10.12}
     ARCH_LIST=""
     NATIVE=
     for arch in $MACOS_REDIST_ARCHS; do