[Cxx] Build the C++ interop support library for the Static SDK for Linux.

We need the C++ interop support library to make the build work, as
the Runtime module uses C++ interop.

(Also, we should build it, because some user programs might want to
use C++ interop.)

rdar://147201087

Author: al45tair

stdlib/public/Cxx/std/CMakeLists.txt

@@ -78,7 +78,7 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_O
     DEPLOYMENT_VERSION_XROS ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_XROS}
 
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
-    TARGET_SDKS ALL_APPLE_PLATFORMS LINUX WINDOWS ANDROID
+    TARGET_SDKS ALL_APPLE_PLATFORMS LINUX LINUX_STATIC WINDOWS ANDROID
     MACCATALYST_BUILD_FLAVOR zippered
     INSTALL_IN_COMPONENT compiler
     INSTALL_BINARY_SWIFTMODULE NON_DARWIN_ONLY