Fix incorrect fread return value check in ilasm strong-name key loading (#121837)

**Summary:**
This PR fixes a logic error in AsmMan::EndAssembly() that incorrectly
treated a successful fread() call as a failure. The incorrect condition:
```
dwBytesRead = fread(m_sStrongName.m_pbPublicKey, 1, m_sStrongName.m_cbPublicKey, fp)) <= m_sStrongName.m_cbPublicKey)
```
causes the error branch to execute even when the full key file has been
read successfully (dwBytesRead == m_sStrongName.m_cbPublicKey).
This results in ilasm failing to read .snk key files when building
TestILAssembly on Big Endian Systems.

**Environment:**
Architecture: s390x
OS: Ubuntu
Runtime : Mono

cc: @uweigand @giritrivedi @saitama951

Author: ShreyaLaxminarayan

src/coreclr/ilasm/asmman.cpp

@@ -437,7 +437,7 @@ void    AsmMan::EndAssembly()
                     // Read the file into the buffer.
                     size_t dwBytesRead;
 
-                    if ((dwBytesRead = fread(m_sStrongName.m_pbPublicKey, 1, m_sStrongName.m_cbPublicKey, fp)) <= m_sStrongName.m_cbPublicKey) {
+                    if ((dwBytesRead = fread(m_sStrongName.m_pbPublicKey, 1, m_sStrongName.m_cbPublicKey, fp)) < m_sStrongName.m_cbPublicKey) {
                         HRESULT hr = HRESULTFromErrno();
                         MAKE_UTF8PTR_FROMWIDE(keySourceNameUtf8, ((Assembler*)m_pAssembler)->m_wzKeySourceName);
                         report->error("Failed to read key file '%s': 0x%d\n",keySourceNameUtf8,hr);