std.os.linux: Define timespec as kernel_timespec (64-bit) for riscv32.

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.
See issue ziglang#4726 for that.

lib/std/os/linux.zig

@@ -6267,12 +6267,13 @@ pub const POSIX_FADV = switch (native_arch) {
 };
 
 /// The timespec struct used by the kernel.
-pub const kernel_timespec = if (@sizeOf(usize) >= 8) timespec else extern struct {
+pub const kernel_timespec = extern struct {
     sec: i64,
     nsec: i64,
 };
 
-pub const timespec = extern struct {
+// TODO: This is a temporary hack until we figure out a coherent Y2038 strategy.
+pub const timespec = if (!builtin.link_libc and native_arch == .riscv32) kernel_timespec else extern struct {
     sec: isize,
     nsec: isize,
 };