tools/nolibc: Remove .global _start from the entry point code

[ Upstream commit 1590c598 ]

Building with clang yields the following error:
```
  <inline asm>:3:1: error: _start changed binding to STB_GLOBAL
  .global _start
  ^
  1 error generated.
```
Make sure only specify one between `.global _start` and `.weak _start`.
Remove `.global _start`.

Cc: llvm@lists.linux.dev
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Stable-dep-of: 184177c3 ("tools/nolibc: restore mips branch ordering in the _start block")
Signed-off-by: Sasha Levin <sashal@kernel.org>