libsa: Remove conflicting .global/.weak directive

LLVM12 complains if you change the symbol binding:
`error: _longjmp changed binding to STB_GLOBAL`
In this case LLVM actually ignored the weak directive and used the
later .global, but GNU as would mark the symbol as weak.
None of the other architectures mark the libsa _setjmp as weak so
just drop this directive.

stand/libsa/amd64/_setjmp.S

@@ -63,7 +63,6 @@ ENTRY(_setjmp)
 	ret
 END(_setjmp)
 
-	.weak	CNAME(_longjmp)
 ENTRY(_longjmp)
 	movq	%rdi,%rdx
 	/* Restore the mxcsr, but leave exception flags intact. */