start_kernel: Omit prevent_tail_call_optimization() for newer toolchains

prevent_tail_call_optimization() was added in
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
to work around stack canaries getting inserted into functions that would
initialize the stack canary in the first place.

Now that we have no_stack_protector function attribute (gcc-11+,
clang-7+) and use it on start_kernel(), remove the call to
prevent_tail_call_optimization() such that we may one day remove it
outright.

Reviewed-by: Nathan Chancellor <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Nick Desaulniers <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

init/main.c

···
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();
 
+	/*
+	 * Avoid stack canaries in callers of boot_init_stack_canary for gcc-10
+	 * and older.
+	 */
+#if !__has_attribute(__no_stack_protector__)
 	prevent_tail_call_optimization();
+#endif
 }
 
 /* Call all constructor functions linked into the kernel. */