Always restore extension methods candidates after each run
now we (can) build Rx assemblies for mobile by *.sln in our mono/rx.git, so remove them from Makefile.
Remove obsolete 32 bit s390 backend.
Support a MONO_ARCH_HAVE_TLS_INIT variable.

Setting this on an architecture makes Mini call
mono_arch_tls_init () on thread attach.

This is primarily useful for architectures that have not
had their LMF code ported to the new infrastructure due to
a general lack of maintenance or hardware to do it on.