Pre-resolve symbols required for the deferred signal processing. This
avoids recursion into rtld when leaving libthr critical section for
the deferred signal delivery.

For the same reason, use syscall(2) instead of referencing
__sys_sigreturn(2). Syscall() is already pre-resolved for fork()
interceptor.

Tested by: Andre Meiser <ortadur@web.de>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week