libc: Add pdfork to the list of interposed system calls

[markj]

Summary

Commmit 21f749da82e7 ("libthr: wrap pdfork(2), same as fork(2).") did
the legwork to implement libthr interposition of pdfork(2), but it seems
that the libc build also needs to be updated to point to the C
implementation. Otherwise the asm stub is used and interposition does
not work.

[markj]

cbdec8db18b5