[libc] Add missing dependencies to RISC-V startup implementation

Summary:
Just like the last patch, the threads and envrion dependencies were
missing. This lead to linker failures when building the tests.

libc/startup/linux/riscv64/CMakeLists.txt

@@ -6,10 +6,12 @@ add_startup_object(
     libc.config.linux.app_h
     libc.include.sys_mman
     libc.include.sys_syscall
+    libc.src.__support.threads.thread
     libc.src.__support.OSUtil.osutil
     libc.src.stdlib.exit
     libc.src.stdlib.atexit
     libc.src.string.memory_utils.memcpy_implementation
+    libc.src.unistd.environ
   COMPILE_OPTIONS
     -fno-omit-frame-pointer
     -ffreestanding # To avoid compiler warnings about calling the main function.