Re-engage SSE2 ASM for SHA-256 (GH# 674)

I'm not usre where exactly the break occured, but it was probably due to the BASE+SIMD rewrite. Thanks to tesbayda for tracking it down.

sha.cpp

@@ -710,7 +710,7 @@ size_t SHA256::HashMultipleBlocks(const word32 *input, size_t length)
         return length & (SHA256::BLOCKSIZE - 1);
     }
 #endif
-#if CRYPTOPP_SSE2_ASM_AVAILABLE
+#if CRYPTOPP_SSE2_ASM_AVAILABLE || CRYPTOPP_X64_MASM_AVAILABLE
     if (HasSSE2())
     {
         const size_t res = length & (SHA256::BLOCKSIZE - 1);

x64dll.asm

@@ -676,7 +676,7 @@ ret
 GCM_AuthenticateBlocks_64K ENDP
 
 ALIGN   8
-SHA256_SSE_HashMultipleBlocks	PROC FRAME
+SHA256_HashMultipleBlocks_SSE2	PROC FRAME
 rex_push_reg rsi
 push_reg rdi
 push_reg rbx
@@ -1962,7 +1962,7 @@ pop		rbx
 pop		rdi
 pop		rsi
 ret
-SHA256_SSE_HashMultipleBlocks ENDP
+SHA256_HashMultipleBlocks_SSE2 ENDP
 
 _TEXT ENDS
 END