posix: Fix internal p{read,write} plt usage

This patch adds internal alias for __pread, __pread64, and __pwrite
following the already in place one for __pwrite64.  This is not used
in any implementation but on microblaze on preadv/pwritev fallback
(since it does not define __ASSUME_PREADV).

In fact it was signaled by commit c35db50f which update the expected
localptl.data for the architecture based on resulted value.  This patch
updates the plt for microblaze now that p{read,write}{64} are correctly
routed to use internal alias.

Checked on x86_64-linux-gnu and a build for all supported architectures
(no all variants although).

	* include/unistd.h (__pread): Add libc_hidden_proto.
	(__pread64): Likewise.
	(__pwrite): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/localplt.data [libc.so]
	(__pread64): Remove.
	* sysdeps/unix/sysv/linux/pread.c (__pread64): Add libc_hidden_weak.
	* sysdeps/unix/sysv/linux/pread64.c (__pread64): Likewise.
	* sysdeps/unix/sysv/linux/pwrite.c (__pwrite): Likewise.