Fixes a bug in 64-bit that led to a non-zero pointer on some systems and a subsequent test failure. Using NULL works for both 32-bit and 64-bit.
Additionally,
Adds missing implicit registers to clobber lists of syscall inline asm.
Adds yet another version for pselect6, in inline asm. The test is semantically redundant to the existing NULL pointer test, and would only expose unexpected changes in glibc's syscall() implementation. Added for completeness.
Changes 2 other long long immediate arguments into passing their natural size.
Fixes i#3911