amd64/bn_arch.h: avoid redefinition of OPENSSL_NO_ASM

Fixes this warning when `OPENSSL_NO_ASM` is already set by the build
system (seen with CMake):
```
In file included from ./libressl/crypto/bn/bn_mul.c:65:
./libressl/crypto/bn/arch/amd64/bn_arch.h:24:9: warning: 'OPENSSL_NO_ASM' macro redefined [-Wmacro-redefined]
        ^
<command line>:10:9: note: previous definition is here
        ^
```

Reported in #910

patches/win32_amd64_bn_arch.h.patch

@@ -1,17 +1,19 @@
 --- crypto/bn/arch/amd64/bn_arch.h.orig	2023-02-27 04:43:12.257874699 -0600
 +++ crypto/bn/arch/amd64/bn_arch.h	2023-02-27 04:44:05.541873870 -0600
-@@ -20,6 +20,10 @@
+@@ -20,6 +20,12 @@
  #ifndef HEADER_BN_ARCH_H
  #define HEADER_BN_ARCH_H
 
 +#ifdef _WIN32
++#ifndef OPENSSL_NO_ASM
 +#define OPENSSL_NO_ASM
++#endif
 +#else
 +
  #ifndef OPENSSL_NO_ASM
 
  #define HAVE_BN_ADD
-@@ -85,6 +89,7 @@
+@@ -85,6 +91,7 @@
  	*out_r0 = r0;
  }
  #endif /* __GNUC__ */