Fixes a bunch of issues.

• Make getrandom work even when called via syscall(2).

• Fix signal-safety of close(2).

• Fix bugs where the llvm optimizer would remove a memory-unsafe operation along with its runtime check.

• Implement pthread_kill.

• Fix a low-probability GC crash that could happen if you combined fork(2) and threads.

• Make threadlocals much faster and give them support for aliases.

• Add .filc_rename and .filc_symver directives for those times when you need to do linker hacks. Also improve .filc_globl.

• Fix a bug where the runtime would crash if it encountered a data relocation that pointed at an unresolved weak symbol.

• Make indirect branches work (i.e. computed goto).

• Implement forced unwinding.

• Implement attribute((cleanup(...))).

• Make it possible to get the frame address from a jmp_buf.