Revert "Retire GCC FORTIFY."

This reverts commit 067bdc9546557a1a107f2617d7d15b864ff493f1.

The angler/bullhead builds still use GCC, so I'll wait until they're turned down.

Bug: https://issuetracker.google.com/74404306
Change-Id: I777b08dd6ab5250d532b4b5cc56e9a790c13db20
diff --git a/libc/include/complex.h b/libc/include/complex.h
index 50da330..3fe8f35 100644
--- a/libc/include/complex.h
+++ b/libc/include/complex.h
@@ -44,9 +44,15 @@
 #define	I		_Complex_I
 
 #if __STDC_VERSION__ >= 201112L
+#ifdef __clang__
 #define	CMPLX(x, y)	((double complex){ x, y })
 #define	CMPLXF(x, y)	((float complex){ x, y })
 #define	CMPLXL(x, y)	((long double complex){ x, y })
+#else
+#define	CMPLX(x, y)	__builtin_complex((double)(x), (double)(y))
+#define	CMPLXF(x, y)	__builtin_complex((float)(x), (float)(y))
+#define	CMPLXL(x, y)	__builtin_complex((long double)(x), (long double)(y))
+#endif
 #endif
 
 __BEGIN_DECLS