um: Fix misaligned stack in stub_exe

The stub_exe could segfault when built with some compilers (e.g. gcc
13.2.0), as SSE instructions which relied on stack alignment could be
generated, but the stack was misaligned.

This seems to be due to the __start entry point being run with a 16-byte
aligned stack, but the x86_64 SYSV ABI wanting the stack to be so
aligned _before_ a function call (so it is misaligned when the function
is entered due to the return address being pushed). The function
prologue then realigns it. Because the entry point is never _called_,
and hence there is no return address, the prologue is therefore actually
misaligning it, and causing the generated movaps instructions to
SIGSEGV. This results in the following error:
start_userspace : expected SIGSTOP, got status = 139

Don't generate this prologue for __start by using
__attribute__((naked)), which resolves the issue.

Fixes: 32e8eaf263 ("um: use execveat to create userspace MMs")
Signed-off-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/linux-um/CABVgOS=boUoG6=LHFFhxEd8H8jDP1zOaPKFEjH+iy2n2Q5S2aQ@mail.gmail.com/
Link: https://patch.msgid.link/20241017231007.1500497-2-davidgow@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/kernel/skas/stub_exe.c

@@ -79,7 +79,7 @@ noinline static void real_init(void)
 	__builtin_unreachable();
 }
 
-void _start(void)
+__attribute__((naked)) void _start(void)
 {
 	char *alloc;