Made External_Primitive_Table::add thread-safe
Made External_Primitive_Table::add thread-safe

- that was most likely never a problem in practices, but there was a race condition that is not obviously avoided by some external guarantees

Signed-off-by: Stefan Marr <46f1a0bd5592a2f9244ca321b129902a06b53e03@stefan-marr.de>

smarr/OmniVM,smarr/OmniVM,smarr/OmniVM,smarr/OmniVM,smarr/OmniVM,smarr/OmniVM,smarr/OmniVM,smarr/OmniVM