[libc][NFC] Adjust use of off_t internally (#68269)

This patch includes changes related to the use of off_t in libc,
targeted at 32-bit systems: in several places, the offset is used either
as a long or an off_t (64-bit signed int), but in 32-bit systems a long
type is only 32 bits long.

Fix a warning in mmap where a long offset is expected, but we were
passing an off_t. A static_cast and a comment were added to explain
that we know we are ignoring the upper 32-bit of the off_t in 32-bit
systems.
The code in pread and pwrite was slightly improved to remove a
#ifdef LIBC_TARGET_ARCH_IS_RISCV32; we are using an if constexpr now.
The Linux file operations were changed to use off_t instead of a long
where applicable. No changes were made to the standard API, e.g.,
ftell returns the offset as an int so we added a static_cast and a
comment explaining that this will cause a loss of integer precision
in 32-bit systems.