[libc][NFC] Adjust use of off_t internally

[mikhailramalho]

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.

1. 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.
2. 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.
3. 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.