tls: P256: sp_256_sub_8_p256_mod always subtracts in-place, use that

i386:

function                                             old     new   delta
sp_256_mont_reduce_8                                 245     243      -2
sp_256_mont_dbl_8                                     26      24      -2
sp_256_ecc_mulmod_8                                 1161    1157      -4
sp_256_proj_point_dbl_8                              359     353      -6
sp_256_sub_8_p256_mod                                 71      32     -39
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/5 up/down: 0/-53)             Total: -53 bytes

non-asm code:

function                                             old     new   delta
sp_256_sub_8_p256_mod                                  -      12     +12
sp_256_mont_reduce_8                                 250     243      -7
sp_256_mont_dbl_8                                     31      24      -7
sp_256_ecc_mulmod_8                                 1171    1157     -14
sp_256_proj_point_dbl_8                              374     353     -21
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/4 up/down: 12/-49)            Total: -37 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed