This is kind of a hack because the timespec in UAPI headers is actually still
32-bit while __kernel_timespec is 64-bit. But, importantly, all the syscalls
take __kernel_timespec from the get-go (because riscv32 support is so recent).

Defining our timespec this way will allow all the syscall wrappers in
std.os.linux to do the right thing for riscv32. For other 32-bit architectures,
we have to use the 64-bit time syscalls explicitly to solve the Y2038 problem.