qmlcachegen fails to build on architectures without JIT

qmlcachegen.cpp tries to use QV4::JIT::createISelForArchitecture function, however the source code of that function is inside #if ENABLE(ASSEMBLER) block, so it is not built on architectures without JIT support.

So on such architectures we get this compilation error:

qmlcachegen.cpp:330: error: undefined reference to 'QV4::JIT::createISelForArchitecture(QString const&)'

The full build logs can be seen here.

I think either there should be some fallback code for architectures without JIT support, or qmlcachegen should not be built on such architectures at all.

Cc shausman because he was the author of https://codereview.qt-project.org/183950.