Remove FIXME question

brson said the answer to the question is probably no... so I'm
going to be bold and remove this.

Closes #2685

Author: catamorphism

src/rt/arch/x86_64/morestack.S

@@ -51,8 +51,6 @@ MORESTACK:
 
         subq $184, %rsp
 
-	// FIXME: libgcc also saves rax. not sure if we need to (#2685)
-
 	// Save argument registers of the original function
 	movq %rdi,       (%rsp)
 	movq %rsi,      8(%rsp)