FSGSBASE and user-controlled TLS
This MR lets userspace manually modify FSBASE and GSBASE on older CPUs via a restricted version of
thisproc:. Newer CPUs can instead use
rdfsbase/rdgsbase, and those instructions are enabled if detected.
Additionally, this removes all kernel handling of thread-local storage, and just like Linux does, lets userspace handle TLS on their own. For this, every time
fexec() is run, there will be a new grant (which the program can later funmap if desired) pointed to by
AT_PHNUM for indexing ELF segments).