[libc][stdfix] disable lkbits on riscv (llvm#115781)

Link: llvm#114912
Link: llvm#115778

Author: nickdesaulniers

libc/config/linux/riscv/entrypoints.txt

@@ -727,7 +727,8 @@ if(LIBC_COMPILER_HAS_FIXED_POINT)
     libc.src.stdfix.uhkbits
     libc.src.stdfix.kbits
     libc.src.stdfix.ukbits
-    libc.src.stdfix.lkbits
+    # TODO: https://github.com/llvm/llvm-project/issues/115778
+    # libc.src.stdfix.lkbits
     libc.src.stdfix.ulkbits
   )
 endif()