Initial port of pthread-embedded to Unikraft

This is our initial port of pthread-embedded to Unikraft as external
library. For now you need newlib to make it work. When adding the
library in the dependency list, pthread-embedded should stay before
newlib (e.g. ...:$(UK_LIBS)/pthread-embedded:$(UK_LIBS)/newlib.git:...).

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Florian Schmidt <florian.schmidt@neclab.eu>

patches: Add patches for proper building of original library

Current patches include bugfixes and refinements in the original code.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Florian Schmidt <florian.schmidt@neclab.eu>

Add .gitignore

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Florian Schmidt <florian.schmidt@neclab.eu>

Add documentation

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Florian Schmidt <florian.schmidt@neclab.eu>

Initial commit (empty)