[release/2.1] Fix build errors in some build configurations

* Change #if NEED_OPENSSL_1_0 to #ifdef NEED_OPENSSL_1_0

This was a mistake in e4bcbd5 (#34443) made when backporting it to
the release branches. All other uses of NEED_OPENSSL_1_0 are guarded by
an #ifdef, not an #if. This one should be too.

The warning is seen when building corefx using source-build. I couldn't observe
it directly:

    source-build/src/corefx/src/Native/Unix/System.Security.Cryptography.Native/pal_asn1_print.cpp:34:5:
    error: 'NEED_OPENSSL_1_0' is not defined, evaluates to 0 [-Werror,-Wundef]
      #if NEED_OPENSSL_1_0
      ^

* Disable used-but-marked-unused warnings in System.Security.Cryptography.Native

When building in non-portable mode, some OpenSSL 1.1 function defnitions
that are marked as unused can be picked up by our build. When those
functions are called, clang reports a warning and fails the build:

    src/Native/Unix/System.Security.Cryptography.Native/openssl.cpp:432:12:
    error: 'sk_ASN1_OBJECT_num' was marked unused but was used [-Werror,-Wused-but-marked-unused]
          return sk_ASN1_OBJECT_num(eku);
                 ^

This 'unused' attribute was recently added to sk_* methods in OpenSSL
1.1: openssl/openssl#8246

src/Native/Unix/System.Security.Cryptography.Native/CMakeLists.txt

@@ -5,6 +5,7 @@ set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 # These are happening inside of OpenSSL-defined macros out of our control
 add_compile_options(-Wno-cast-align)
+add_compile_options(-Wno-used-but-marked-unused)
 
 add_definitions(-DPIC=1 -DOPENSSL_API_COMPAT=0x10100000L)
 

src/Native/Unix/System.Security.Cryptography.Native/pal_asn1_print.cpp

@@ -31,7 +31,7 @@ static_assert(PAL_ASN1_STRFLGS_UTF8_CONVERT == ASN1_STRFLGS_UTF8_CONVERT, "");
 
 extern "C" ASN1_STRING* CryptoNative_DecodeAsn1TypeBytes(const uint8_t* buf, int32_t len, Asn1StringTypeFlags type)
 {
-#if NEED_OPENSSL_1_0
+#ifdef NEED_OPENSSL_1_0
     if (!API_EXISTS(d2i_ASN1_type_bytes) || !buf || !len)
     {
         return nullptr;