risc-v/cmake: set nostdlib to c compiler

To avoid build break:

ld: riscv-none-elf/lib/rv64imafdc_zicsr/lp64d/crt0.o: in function `.L0 ':
(.text+0x8): undefined reference to `__bss_start'
ld: (.text+0x10): undefined reference to `_end'
ld: (.text+0x36): undefined reference to `main'
collect2: error: ld returned 1 exit status

Signed-off-by: chao an <anchao@lixiang.com>

arch/risc-v/src/cmake/Toolchain.cmake

@@ -145,7 +145,7 @@ if(CONFIG_DEBUG_OPT_UNUSED_SECTIONS)
   add_compile_options(-ffunction-sections -fdata-sections)
 endif()
 
-add_link_options(-Wl,-nostdlib)
+add_link_options(-nostdlib)
 add_link_options(-Wl,--entry=__start)
 
 if(CONFIG_DEBUG_LINK_MAP)