x86-urgent-2020-11-08

 - Use SYM_FUNC_START_WEAK in the mem* ASM functions instead of a
   combination of .weak and SYM_FUNC_START_LOCAL which makes LLVMs
   integrated assembler upset.

 - Correct the mitigation selection logic which prevented the related prctl
   to work correctly.

 - Make the UV5 hubless system work correctly by fixing up the malformed
   table entries and adding the missing ones.