[libc] Add support to compile some syscalls on 32 bit platform

This patch adds a bunch of ifdefs to handle the 32 bit versions of
some syscalls, which often only append a 64 to the name of the syscall
(with exception of SYS_lseek -> SYS_llseek and SYS_futex ->
SYS_futex_time64)

This patch also tries to handle cases where wait4 is not available
(as in riscv32): to implement wait, wait4 and waitpid when wait4 is
not available, we check for alternative wait calls and ultimately rely
on waitid to implement them all.

In riscv32, only waitid is available, so we need it to support this
platform.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148371