test(asm): fuzz the parse-and-assemble pipeline for amd64

Assisted-by: GLM 5.3 Flash

asm/testdata/include/fuzzdefs.h

@@ -0,0 +1,5 @@
+// Fixture for the assembler fuzz target: a constant macro and a
+// parameterised one, so the corpus's #include seeds splice real
+// definitions the way go_tls.h and textflag.h do in production sources.
+#define KONST $42
+#define ARG(n) n+0(FP)