libc: fix mips N64 pread/pwrite build

Uclibc is not building for MIPS N64 because pread is trying to use the
pread/pwrite system calls instead of pread64/pwrite64.  This patch fixes
the problem and was tested with LFS enabled and disabled.

Signed-off-by: Steve Ellcey <sellcey@mips.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>