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: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5708)