linux-user: syscall.c lacks pselect6_time64

in commit 50efc69586388a975c1ebd90cb8cc8e4a7328bc4 legacy pselect6 definition
for riscv32 was removed in favour of pselect6_time64, but pselect6_time64 is
not available in syscall.c, thus leaving riscv32 without pselect syscall.