Assume prlimit64 is available.

This patch makes sysdeps/unix/sysv/linux code assume the prlimit64
syscall is always available, given the minimum of a 3.2 kernel.

__ASSUME_PRLIMIT64, which in fact was no longer used, is removed.
Code conditional on __NR_prlimit64 being defined is made
unconditional.  Fallback code for the case where prlimit64 produces an
ENOSYS error is removed, substantially simplifying some functions.

Tested for x86_64 and x86.

	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_PRLIMIT64):
	Remove macro.
	* sysdeps/unix/sysv/linux/getrlimit64.c (__getrlimit64): Assume
	prlimit64 is always available and does not give an ENOSYS error.
	* sysdeps/unix/sysv/linux/prlimit.c [__NR_prlimit64]: Make code
	unconditional.
	[!__NR_prlimit64]: Remove conditional code.
	* sysdeps/unix/sysv/linux/setrlimit.c (__setrlimit): Assume
	prlimit64 is always available and does not give an ENOSYS error.
	* sysdeps/unix/sysv/linux/setrlimit64.c (__setrlimit64): Likewise.