fix: do not define _LIBCPP_ABI_NAMESPACE=Cr for all native modules (#34932)

This define is only needed when linking against Chromiums libc++ which we currently
do not ship / expose the symbols of.  We probably should make those symbols visible and
actually ensure that electron-rebuild et. al link against our libc++ instead of the system libc++
but for now this fixes compilation issues on macOS where the default system clang links to the system libc++
which does not (obviously) use the Chromium ABI namespace.

For our nan tests which do link against Chromiums libc++ we define the ABI namespace in the spec runner.

script/nan-spec-runner.js

@@ -63,6 +63,7 @@ async function main () {
     `-isystem"${path.resolve(BASE, 'buildtools', 'third_party', 'libc++', 'trunk', 'include')}"`,
     `-isystem"${path.resolve(BASE, 'buildtools', 'third_party', 'libc++abi', 'trunk', 'include')}"`,
     '-fPIC',
+    '-D_LIBCPP_ABI_NAMESPACE=Cr',
     ...platformFlags
   ].join(' ');