Fix popen_test in MODE=dbg

ASAN and vfork() don't appear to play well together. Maybe in later
versions of GCC it'll be better. But vfork() is flirting with danger
after all and that probably doesn't make sense in ASAN mode anyway.

libc/runtime/vfork.S

@@ -20,6 +20,11 @@
 #include "libc/macros.internal.h"
 .privileged
 
+#ifdef __FSANITIZE_ADDRESS__
+vfork:	jmp	fork			# TODO: asan and vfork don't mix?
+	.endfn	vfork,globl
+#else
+
 //	Forks process without copying page tables.
 //
 //	This is the same as fork() except it's optimized for the case
@@ -75,3 +80,5 @@ vfork.bsd:
 	jmp	0b
 	.endfn	vfork.bsd
 #endif /* BSD */
+
+#endif /* __FSANITIZE_ADDRESS__ */