36 hours of building, 4 GiB ram, 7GiB swap and I get

 |  /usr/bin/ld: final link failed: memory exhausted
 |  collect2: error: ld returned 1 exit status
 |  [2908/2932] Building CXX object Source/WebCore/CMakeFiles/WebCoreTestSupport.dir/__/__/DerivedSources/WebCore/JSMockPageOverlay.cpp.o
 |  DerivedSources/WebCore/JSMockPageOverlay.cpp: In member function ‘virtual bool WebCore::JSMockPageOverlayOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void*, JSC::SlotVisitor&)’:
 |  DerivedSources/WebCore/JSMockPageOverlay.cpp:140:6: note: parameter passing for argument of type ‘JSC::Handle<JSC::Unknown>’ changed in GCC 7.1
 |   bool JSMockPageOverlayOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
 |        ^~~~~~~~~~~~~~~~~~~~~~
 |  DerivedSources/WebCore/JSMockPageOverlay.cpp: In member function ‘virtual void WebCore::JSMockPageOverlayOwner::finalize(JSC::Handle<JSC::Unknown>, void*)’:
 |  DerivedSources/WebCore/JSMockPageOverlay.cpp:147:6: note: parameter passing for argument of type ‘JSC::Handle<JSC::Unknown>’ changed in GCC 7.1
 |   void JSMockPageOverlayOwner::finalize(JSC::Handle<JSC::Unknown> handle, void* context)
 |        ^~~~~~~~~~~~~~~~~~~~~~
 |  [2909/2932] Building CXX object Source/WebCore/CMakeFiles/WebCoreTestSupport.dir/__/__/DerivedSources/WebCore/JSInternals.cpp.o
 |  DerivedSources/WebCore/JSInternals.cpp: In member function ‘virtual bool WebCore::JSInternalsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown>, void*, JSC::SlotVisitor&)’:
 |  DerivedSources/WebCore/JSInternals.cpp:8449:6: note: parameter passing for argument of type ‘JSC::Handle<JSC::Unknown>’ changed in GCC 7.1
 |   bool JSInternalsOwner::isReachableFromOpaqueRoots(JSC::Handle<JSC::Unknown> handle, void*, SlotVisitor& visitor)
 |        ^~~~~~~~~~~~~~~~
 |  DerivedSources/WebCore/JSInternals.cpp: In member function ‘virtual void WebCore::JSInternalsOwner::finalize(JSC::Handle<JSC::Unknown>, void*)’:
 |  DerivedSources/WebCore/JSInternals.cpp:8456:6: note: parameter passing for argument of type ‘JSC::Handle<JSC::Unknown>’ changed in GCC 7.1
 |   void JSInternalsOwner::finalize(JSC::Handle<JSC::Unknown> handle, void* context)
 |        ^~~~~~~~~~~~~~~~
 |  ninja: build stopped: subcommand failed.
 |  ==> ERROR: A failure occurred in build().
 |      Aborting...
==> Copying log and package files out of the chroot...

The above error is due to some issue with doing debug builds in a cross-compile. Which causes errors like
/usr/bin/ld.gold: fatal error: lib/libwebkit2gtk-4.0.so.37.33.5: mmap: failed to allocate 1781955648 bytes for output file: Cannot allocate memory'
Even on BeefCake which has massive memory resources.

Will try this again (with debug turned off) once the current iceape build finishes