[Static SDK] Don't build FoundationMacros. (#419)

We don't want to be trying to build FoundationMacros here.  Instead,
just use the one that comes with the compiler we installed.

rdar://136278412

Author: al45tair

swift-ci/sdks/static-linux/scripts/build.sh

@@ -739,6 +739,7 @@ EOF
           -D_SwiftFoundation_SourceDIR=${source_dir}/swift-project/swift-foundation \
           -D_SwiftFoundationICU_SourceDIR=${source_dir}/swift-project/swift-foundation-icu \
           -D_SwiftCollections_SourceDIR=${source_dir}/swift-project/swift-collections \
+          -DSwiftFoundation_MACRO=/usr/local/swift/lib/swift/host/plugins/libFoundationMacros.so
           -DCMAKE_Swift_COMPILER_WORKS=YES \
           -Ddispatch_DIR=${build_dir}/$arch/dispatch/cmake/modules