Consolidate Linux readahead implementation

This patch consolidate Linux readahead implementation on generic
sysdeps/unix/sysv/linux/readahead.c one.  The changes are:

  - Assume __NR_readahead existence with current minimum kernel of 3.2
    for all architectures.

  - Use INLINE_SYSCALL_CALL, __ALIGNMENT_ARG, and SYSCALL_LL64 to pass
    the 64 bit offset.  This allows architectures with different abis
    to use the same implementation.

  - Remove arch-specific readahead implementations.

Checked on x86_64-linux-gnu and i686-linux-gnu.

	* sysdeps/unix/sysv/linux/arm/readahead.c: Remove file.
	* sysdeps/unix/sysv/linux/mips/mips32/readahead.c: Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/n32/syscalls.list (readahead):
	Remove.
	* sysdeps/unix/sysv/linux/mips/mips64/n64/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/wordsize-64/syscalls.list: Likewise.
	* sysdeps/unix/sysv/linux/readahead.c (__readahead): Assume
	__NR_readahead existence, and use INLINE_SYSCALL_CALL, __ALIGNMENT_ARG,
	and SYSCALL_LL64.