Remove __ASSUME_FALLOCATE.

Given current Linux kernel version requirements, we can always assume
the fallocate syscall to be available.  This patch removes
__ASSUME_FALLOCATE and a test for whether __NR_fallocate is defined.

Tested for x86_64 and x86 that installed stripped shared libraries are
unchanged by the patch.

	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_FALLOCATE):
	Remove macro.
	* sysdeps/unix/sysv/linux/wordsize-64/posix_fallocate.c: Do not
	include <kernel-features.h>.
	[!__ASSUME_FALLOCATE]: Remove conditional code.
	(posix_fallocate) [__NR_fallocate]: Make code unconditional.