always use __errno_location for user ABI

Makefile

@@ -475,6 +475,7 @@ endif
 endif
 endif
 
+CFLAGS += -D_BUILDING_WASI_LIBC
 ifeq ($(WASI_SNAPSHOT), p2)
 CFLAGS += -D__wasilibc_use_wasip2
 endif
@@ -971,7 +972,7 @@ check-symbols: $(STARTUP_FILES) libc
 	@# TODO: Undefine __wasm_nontrapping_fptoint__, __wasm_bulk_memory__ and
 	@# __wasm_bulk_memory_opt__, that are new to clang 20.
 	@# TODO: As of clang 16, __GNUC_VA_LIST is #defined without a value.
-	$(CC) $(CFLAGS) "$(SYSROOT_SHARE)/include-all.c" \
+	$(CC) $(CFLAGS) -U_BUILDING_WASI_LIBC "$(SYSROOT_SHARE)/include-all.c" \
 	    -isystem $(SYSROOT_INC) \
 	    -std=gnu17 \
 	    -E -dM -Wno-\#warnings \

expected/wasm32-wasip1-threads/predefined-macros.txt

@@ -3255,7 +3255,7 @@
 #define dirent64 dirent
 #define erf(x) __tg_real(erf, (x))
 #define erfc(x) __tg_real(erfc, (x))
-#define errno errno
+#define errno (*__errno_location())
 #define exp(x) __tg_real_complex(exp, (x))
 #define exp2(x) __tg_real(exp2, (x))
 #define expm1(x) __tg_real(expm1, (x))

expected/wasm32-wasip1/predefined-macros.txt

@@ -3250,7 +3250,7 @@
 #define dirent64 dirent
 #define erf(x) __tg_real(erf, (x))
 #define erfc(x) __tg_real(erfc, (x))
-#define errno errno
+#define errno (*__errno_location())
 #define exp(x) __tg_real_complex(exp, (x))
 #define exp2(x) __tg_real(exp2, (x))
 #define expm1(x) __tg_real(expm1, (x))

expected/wasm32-wasip2/predefined-macros.txt

@@ -3403,7 +3403,7 @@
 #define dirent64 dirent
 #define erf(x) __tg_real(erf, (x))
 #define erfc(x) __tg_real(erfc, (x))
-#define errno errno
+#define errno (*__errno_location())
 #define exp(x) __tg_real_complex(exp, (x))
 #define exp2(x) __tg_real(exp2, (x))
 #define expm1(x) __tg_real(expm1, (x))

libc-bottom-half/headers/public/__errno.h

@@ -5,9 +5,15 @@
 extern "C" {
 #endif
 
+#ifdef _BUILDING_WASI_LIBC
+/* libc internal access */
 extern _Thread_local int errno;
-
 #define errno errno
+#else
+/* user exposed ABI */
+__attribute__((__const__)) int *__errno_location(void);
+#define errno (*__errno_location())
+#endif
 
 #ifdef __cplusplus
 }