Add bignum_{sqr,mul}_p521_neon

This adds `bignum_{sqr,mul}_p521_neon` and their proofs.

The new subroutine specs are added to specification.txt, and test as well as benchmark are updated.

Benchmark results on GV2 are:

```
bignum_mul_p521                 :   135.1 ns each (var  0.2%, corr -0.01) =    7404184 ops/sec
bignum_mul_p521_neon            :   115.5 ns each (var  0.3%, corr  0.00) =    8660108 ops/sec
bignum_sqr_p521                 :   108.9 ns each (var  0.2%, corr  0.08) =    9184994 ops/sec
bignum_sqr_p521_neon            :    78.7 ns each (var  0.3%, corr  0.06) =   12708368 ops/sec
```

.gitignore

@@ -2,6 +2,7 @@
 *.correct
 *.o
 *.obj
+.vscode
 tests/test
 tests/ctCheck
 benchmarks/benchmark

arm/Makefile

@@ -308,10 +308,12 @@ BIGNUM_OBJ = curve25519/bignum_add_p25519.o \
              p521/bignum_montsqr_p521_neon.o \
              p521/bignum_mul_p521.o \
              p521/bignum_mul_p521_alt.o \
+             p521/bignum_mul_p521_neon.o \
              p521/bignum_neg_p521.o \
              p521/bignum_optneg_p521.o \
              p521/bignum_sqr_p521.o \
              p521/bignum_sqr_p521_alt.o \
+             p521/bignum_sqr_p521_neon.o \
              p521/bignum_sub_p521.o \
              p521/bignum_tolebytes_p521.o \
              p521/bignum_tomont_p521.o \

arm/p521/Makefile

@@ -38,10 +38,12 @@ OBJ = bignum_add_p521.o \
       bignum_montsqr_p521_neon.o \
       bignum_mul_p521.o \
       bignum_mul_p521_alt.o \
+      bignum_mul_p521_neon.o \
       bignum_neg_p521.o \
       bignum_optneg_p521.o \
       bignum_sqr_p521.o \
       bignum_sqr_p521_alt.o \
+      bignum_sqr_p521_neon.o \
       bignum_sub_p521.o \
       bignum_tolebytes_p521.o \
       bignum_tomont_p521.o \

arm/p521/bignum_montsqr_p521_neon.S

@@ -562,7 +562,7 @@
 // The bash script used for step 2 is as follows:
 //
 //        # Store the assembly instructions except the last 'ret',
-//        # callee-register store/loads and add/sub sp #80 as, say, 'input.S'.
+//        # callee-register store/loads as, say, 'input.S'.
 //        export OUTPUTS="[hint_buffer0,hint_buffer16,hint_buffer32,hint_buffer48,hint_buffer64]"
 //        export RESERVED_REGS="[x18,x25,x26,x27,x28,x29,x30,sp,q8,q9,q10,q11,q12,q13,q14,q15,v8,v9,v10,v11,v12,v13,v14,v15]"
 //        <s2n-bignum>/tools/external/slothy.sh input.S my_out_dir