objtool, kcsan: Add explicit check functions to uaccess whitelist

Add explicitly invoked KCSAN check functions to objtool's uaccess
whitelist. This is needed in order to permit calling into
kcsan_check_scoped_accesses() from the fast-path, which in turn calls
__kcsan_check_access().  __kcsan_check_access() is the generic variant
of the already whitelisted specializations __tsan_{read,write}N.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>