Adds handling for pselect6_time64 same as the existing handling for pselect6. This is still conditionally compiled based on whether SYS_pselect6_time64 is defined, because the *_time64 syscalls are defined only on 32-bit.
Adds a test for pselect6_time64 with non-null sigmask that fails without this. The one with null sigmask doesn't really need special handling but added it anyway.
Adds a test for ppoll_time64 with non-null sigmask that fails without the ppoll_time64 handling. The existing one for ppoll_time64 was with null sigmask. The existing non-null sigmask test for ppoll also does fail without the ppoll_time64 handling, but good to have another one specific to ppoll_time64.
Makes some readability improvements in the test source: removes some empty lines that weren't helping, and rearranges some code to bring declaration and use closer for some variables.
Issue: #5131 (closed)