Fix P-384 curve on lower-than-P9 PPC64 targets

Fix P-384 curve on lower-than-P9 PPC64 targets

The change adding an asm implementation of p384_felem_reduce incorrectly
uses the accelerated version on both targets that support the intrinsics
*and* targets that don't, instead of falling back to the generics on older
targets.  This results in crashes when trying to use P-384 on < Power9.

Signed-off-by: Anna Wilcox <AWilcox@Wilcox-Tech.com>
Closes: #27350
Fixes: 85cabd94 ("Fix Minerva timing side-channel signal for P-384 curve on PPC")
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27429)