Bifurcate microarchitectural variants of Weierstrass point additions

As with the earlier update for doublings, the Jacobian point adidtion
and mixed addition operations for the curves P-256, P-384, P-521,
secp256k1 and SM2 now all have the usual two versions targeting
different microarchitectures, one of them called "_alt", following the
general s2n-bignum convention.

The "_alt" forms for ARM now present are just renamed versions of the
originals (which were based on "_alt" field operations), with the new
code taking over the old non-alt name. For x86 the non-alt ones are
the same as before and the "_alt" forms are new.

arm/Makefile

@@ -92,25 +92,35 @@ POINT_OBJ = curve25519/curve25519_ladderstep.o \
             curve25519/edwards25519_scalarmuldouble.o \
             curve25519/edwards25519_scalarmuldouble_alt.o \
             p256/p256_montjadd.o \
+            p256/p256_montjadd_alt.o \
             p256/p256_montjdouble.o \
             p256/p256_montjdouble_alt.o \
             p256/p256_montjmixadd.o \
+            p256/p256_montjmixadd_alt.o \
             p384/p384_montjadd.o \
+            p384/p384_montjadd_alt.o \
             p384/p384_montjdouble.o \
             p384/p384_montjdouble_alt.o \
             p384/p384_montjmixadd.o \
+            p384/p384_montjmixadd_alt.o \
             p521/p521_jadd.o \
+            p521/p521_jadd_alt.o \
             p521/p521_jdouble.o \
             p521/p521_jdouble_alt.o \
             p521/p521_jmixadd.o \
+            p521/p521_jmixadd_alt.o \
             secp256k1/secp256k1_jadd.o \
+            secp256k1/secp256k1_jadd_alt.o \
             secp256k1/secp256k1_jdouble.o \
             secp256k1/secp256k1_jdouble_alt.o \
             secp256k1/secp256k1_jmixadd.o \
+            secp256k1/secp256k1_jmixadd_alt.o \
             sm2/sm2_montjadd.o \
+            sm2/sm2_montjadd_alt.o \
             sm2/sm2_montjdouble.o \
             sm2/sm2_montjdouble_alt.o \
-            sm2/sm2_montjmixadd.o
+            sm2/sm2_montjmixadd.o \
+            sm2/sm2_montjmixadd_alt.o
 
 BIGNUM_OBJ = curve25519/bignum_add_p25519.o \
              curve25519/bignum_cmul_p25519.o \

arm/p256/Makefile

@@ -47,9 +47,11 @@ OBJ = bignum_add_p256.o \
       bignum_tomont_p256.o \
       bignum_triple_p256.o \
       p256_montjadd.o \
+      p256_montjadd_alt.o \
       p256_montjdouble.o \
       p256_montjdouble_alt.o \
-      p256_montjmixadd.o
+      p256_montjmixadd.o \
+      p256_montjmixadd_alt.o
 
 %.o : %.S ; $(CC) -E -I../../include $< | $(GAS) -o $@ -