tools/nolibc/arch: mark the _start symbol as weak

[ Upstream commit dffeb81a ]

By doing so we can link together multiple C files that have been compiled
with nolibc and which each have a _start symbol.

Signed-off-by: Willy Tarreau <w@1wt.eu>
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>