test: port the @symbol FFI observer to ww; retire 800_ffi

Strengthens the pre-symmetry-era cstage-only stdout scrape to both
frontends with gated exits and the rule-10 cs==ww byte-id; the libc
binding smoke likewise runs both stages.

test/wcc/800_ffi.c

@@ -1,81 +0,0 @@
-/*
- * 800_ffi — verify the @symbol("X") attribute redirects CALLs at
- * the codegen layer. The ww source declares a body-less fn with an
- * external symbol name, calls it; the emitted .s must reference the
- * external name, not the ww-side ident.
- *
- * Also smoke-tests building lib/c/libc/libc.ww (parse + check + cgen
- * succeed; nothing to run).
- */
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <unistd.h>
-#include "wwtestpkg.h"
-
-static char *
-run6c(const char *bin, const char *src)
-{
-	char path[64];
-	snprintf(path, sizeof path, "/tmp/wwffi_%d.ww", getpid());
-	FILE *f = fopen(path, "wb");
-	wwtest_fputs(src, f);
-	fclose(f);
-
-	char cmd[512];
-	snprintf(cmd, sizeof cmd, "%s/w6c %s 2>&1", bin, path);
-	FILE *p = popen(cmd, "r");
-	size_t cap = 4096, n = 0;
-	char *buf = malloc(cap);
-	int c;
-	while ((c = fgetc(p)) != EOF) {
-		if (n + 1 >= cap) { cap *= 2; buf = realloc(buf, cap); }
-		buf[n++] = (char)c;
-	}
-	buf[n] = '\0';
-	pclose(p);
-	unlink(path);
-	return buf;
-}
-
-int
-main(void)
-{
-	const char *bin = getenv("BIN");
-	if (!bin) bin = "out/bin";
-
-	int fail = 0;
-
-	/* @symbol redirects ident name to symbol name */
-	{
-		char *out = run6c(bin,
-		    "@symbol(\"cout\") fn writeit(b: *u8, n: u64) i64;\n"
-		    "fn main() i32 = { writeit(nil, 0); return 0; };");
-		if (strstr(out, "CALL\tcout(SB)") == NULL) {
-			fprintf(stderr, "ffi: missing CALL cout(SB) in:\n%s\n", out);
-			fail++;
-		}
-		if (strstr(out, "CALL\twriteit(SB)") != NULL) {
-			fprintf(stderr, "ffi: should not call by ident name:\n%s\n", out);
-			fail++;
-		}
-		free(out);
-	}
-
-	/* lib/c/libc parses + cgens (no defined fns means no TEXT directives) */
-	{
-		char path[256];
-		snprintf(path, sizeof path, "%s/../../lib/c/libc/libc.ww", bin);
-		char cmd[512];
-		snprintf(cmd, sizeof cmd, "%s/w6c %s > /dev/null 2>&1", bin, path);
-		int rc = system(cmd);
-		if (rc != 0) {
-			fprintf(stderr, "ffi: w6c failed on libc.ww (rc=%d)\n", rc);
-			fail++;
-		}
-	}
-
-	if (fail) return 1;
-	puts("ffi: ok");
-	return 0;
-}