[libc] make off_t 32b for 32b arm (#77350)

Fixes the following diagnostic:

    llvm-project/libc/src/sys/mman/linux/mmap.cpp:44:59: error: implicit
conversion loses integer precision: 'off_t' (aka 'long long') to 'long'
    [-Werror,-Wshorten-64-to-32]
     size, prot, flags, fd, offset);
                            ^~~~~~

It looks like off_t is a curious types on different platforms. FWICT,
it's 32b
on arm (at least for arm-linux-gnueabi) but 64b elsewhere (including 32b
riscv32-linux-gnu).