[libc] Fix set_thread_ptr call in rv32 start up code

This patch changes the instruction in set_thread_ptr from ld to mv,
as rv32 doesn't have the ld instruction, and mv is supported by both
rv32 and rv64.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159110

libc/startup/linux/riscv64/start.cpp

@@ -80,7 +80,7 @@ void cleanup_tls(uintptr_t addr, uintptr_t size) {
 }
 
 static void set_thread_ptr(uintptr_t val) {
-  LIBC_INLINE_ASM("ld tp, %0\n\t" : : "m"(val));
+  LIBC_INLINE_ASM("mv tp, %0\n\t" : : "r"(val));
 }
 
 using InitCallback = void(int, char **, char **);