runtime/unix: simplify time functions

[aykevl]

There is no need for separate datatypes for 32-bit and 64-bit *nix systems, because the int type already provides this.

See #710

[deadprogram]

✂️

[niaow]

Oh right I forgot we dont do 32-bit x86, so this works.

[aykevl]

Ah, x32 is indeed an exception. However, it isn't even supported by the Go toolchain (GOARCH=amd64p32 was only used with GOOS=nacl, both are now removed) and in general seems to be largely unused nowadays. So hopefully we can ignore this exception, at least until AArch64 ILP32 becomes necessary (and probably even then).