Remove __ASSUME_OFF_DIFF_OFF64 definition

Remove __ASSUME_OFF_DIFF_OFF64 definition

This patch removes the __ASSUME_OFF_DIFF_OFF64 define introduced in
p{read,write} consolidation patch.  This define was added based on
the idea 32 bits ports would continue to follow previous off{64}_t
definition where off_t size differs from off64_t one.

However, with recent AArch64/ILP32 patch submission and also with
discussion for RISCV kernel interface, 32 bits ports now may aim
to use off_t and off64_t with the same size as 64 bits.

So current assumption for both p{read,write} and p{read,write}v
are not compatible with new type definition.  This patch now makes
the syscall wrappers to only depend on __OFF_T_MATCHES_OFF64_T to
define the default and 64-suffix variant, as follow:

  <function>.c
  #ifndef __OFF_T_MATCHES_OFF64_T
  /* build <function> */
  #endif

  and

  <function>64.c

  /* build <function>64 */
  #ifdef __OFF_T_MATCHES_OFF64_T
  weak_alias (fallocate64, fallocate)
  #endif

Tested on x86_64, i686, x32, and armhf.

	* sysdeps/unix/sysv/linux/mips/kernel-features.h
	(__ASSUME_OFF_DIFF_OFF64): Remove define.
	* sysdeps/unix/sysv/linux/pread.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pread): Replace by
	__OFF_T_MATCHES_OFF64_T.
	* sysdeps/unix/sysv/linux/pread64.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pread64): Likewise.
	* sysdeps/unix/sysv/linux/preadv.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (preadv): Likewise.
	* sysdeps/unix/sysv/linux/preadv64.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (preadv64): Likewise.
	* sysdeps/unix/sysv/linux/pwrite.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pwrite): Likewise.
	* sysdeps/unix/sysv/linux/pwrite64.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pwrite64): Likewise.
	* sysdeps/unix/sysv/linux/pwritev.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pwritev): Likewise.
	* sysdeps/unix/sysv/linux/pwritev64.c
	[__WORDSIZE != 64 || __ASSUME_OFF_DIFF_OFF64] (pwritev64): Likewise.