Don't build and install libcxx when generating a macOS toolchain

Libcxx is already part of the macOS SDKs shipped since Xcode 12.5, so
there is no need for us to rebuild it.
This will also prevent compilation issues in the presets that builds the
standard library using a prebuilt toolchain, so the clang compiler that
ships in that toolchain does not pickup libcxx headers that do not
match the tbd shipped in the underlying macOS SDK.

This change would be desirable for all macOS presets, focusing for now
on the toolchain presets.

Addresses rdar://106817494

Author: edymtt

utils/build-presets.ini

@@ -1265,7 +1265,6 @@ swift-driver
 swiftsyntax
 swiftformat
 playgroundsupport
-libcxx
 indexstore-db
 sourcekit-lsp
 swiftdocc
@@ -1308,7 +1307,6 @@ install-swiftpm
 install-swift-driver
 install-swiftsyntax
 install-playgroundsupport
-install-libcxx
 install-sourcekit-lsp
 install-swiftformat
 install-swiftdocc