curve448/field.h: relax alignment, as it doesn't work universally.

Some platforms, cough-DJGPP, fail to compile claiming that requested
alignment is greater than maximum possible. Supposedly original
alignment was result of an attempt to utilize AVX2...
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5708)

crypto/ec/curve448/field.h

@@ -25,7 +25,7 @@
 # if defined(__GNUC__) || defined(__clang__)
 #  define INLINE_UNUSED __inline__ __attribute__((__unused__,__always_inline__))
 #  define RESTRICT __restrict__
-#  define ALIGNED __attribute__((__aligned__(32)))
+#  define ALIGNED __attribute__((__aligned__(16)))
 # else
 #  define INLINE_UNUSED ossl_inline
 #  define RESTRICT