[libc] Unify lseek implementations

In patch D157792, the calls to SYS_llseek/SYS_llseek for 32-bit systems
were fixed in lseek.cpp but there was another implementation in file.cpp
that was missed.

To reduce the code duplication, this patch unifies both call sites to
use a new lseekimpl function.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159208