Avoid unncessary use of double in the kernel. With this change,
a LLVM build kernel fully works on TWRP1025.
Fix c&p error: libisc.a->libirs.a
In case of no dup2'd fd's, make sure that F_CLOSEM for the
rump kernel starts from 0.

Fixes rumphijack fdoff test (notably, this bug had nothing to do with
fdoff, and was exposed >3 years after writing the test when rump kernels
started providing fd's 0/1/2)
Update hardcoded fd expectations now that rump kernels open 0/1/2.
(since we exec, it's convenient to just assume the values of first 4 fds)