libthr: switch thread and sleepq memory allocator to crt from libc malloc

There are more complex interactions between malloc and libthr
initialization that can happen if libthr functions are called from ELF
object' constructors, before libthr is initialized. Break the
dependencies loop by using the private allocator with controlled init.

Reported by: yuri
Reviewed by: markj, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48454