fix(build): Disable LTO on riscv64 to fix linker error (#458)

### Description

This PR fixes a build failure encountered when cross-compiling for the
`riscv64` architecture.

When LTO is enabled (via `-flto=auto`), the build fails during the final
linking stage with the following error:

```

/data/hebo/toolchain/gcc-15.1/lib/gcc/riscv64-unknown-linux-gnu/15.1.0/../../../../riscv64-unknown-linux-gnu/bin/ld: ... hidden symbol \`\_ZN8nanobind11chain\_errorENS\_6handleEPKcz' isn't defined
/data/hebo/toolchain/gcc-15.1/lib/gcc/riscv64-unknown-linux-gnu/15.1.0/../../../../riscv64-unknown-linux-gnu/bin/ld: final link failed: bad value

```

This appears to be caused by the RISC-V toolchain's LTO implementation
incorrectly discarding the `nanobind::chain_error` symbol, which is
required by the bindings.

This patch resolves the issue by modifying the CMake build logic to
conditionally add the `-flto=auto` flag to `CMAKE_CXX_FLAGS` *only* when
the target architecture (`CMAKE_SYSTEM_PROCESSOR`) is not `riscv64`.

### How This Was Tested

The fix has been successfully tested in the following environments:

1. RISC-V 64-bit (Problem Environment):
    * Platform: SG2044
    * Toolchain: GCC 15.1
    * Python: 3.11
    * Result: Build SUCCESS, previously FAILED

2. x86_64:
    * Platform: x86_64
    * Toolchain: GCC 14
    * Python: 3.12
    * Result: Build SUCCESS, LTO remains enabled and functional

Signed-off-by: lyd1992 <[email protected]>
Signed-off-by: ihb2032 <[email protected]>
Co-authored-by: Yixin Dong <[email protected]>

Author: ihb2032

CMakeLists.txt

@@ -51,10 +51,16 @@ else()
   if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
     set(CMAKE_CXX_FLAGS "-O3 ${CMAKE_CXX_FLAGS}")
   endif()
-
-  set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -Wno-pedantic -Wno-unused-parameter \
+  if(CMAKE_SYSTEM_PROCESSOR STREQUAL "riscv64")
+    message(STATUS "RISC-V 64-bit target detected, disabling LTO to prevent linker errors.")
+    set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -Wno-pedantic -Wno-unused-parameter \
+-Woverloaded-virtual ${CMAKE_CXX_FLAGS}"
+    )
+  else()
+    set(CMAKE_CXX_FLAGS "-Wall -Wextra -Werror -Wno-pedantic -Wno-unused-parameter \
 -Woverloaded-virtual -flto=auto ${CMAKE_CXX_FLAGS}"
-  )
+    )
+  endif()
 endif()
 
 set(XGRAMMAR_INCLUDE_PATH ${PROJECT_SOURCE_DIR}/3rdparty/picojson