Use correct includes in benchtests

Currently the benchtests are run with internal GLIBC headers, which is incorrect.
Defining _ISOMAC in the makefile ensures the internal headers are bypassed.
Fix all tests which were relying on internal defines or includes.

	* benchtests/Makefile: Define _ISOMAC.
	* benchtests/bench-strcoll.c: Add missing sys/stat.h include.
	* benchtests/bench-string.h: Define inhibit_loop_to_libcall macro.
	* benchtests/bench-strstr.c: Define empty libc_hidden_builtin_def.
	* benchtests/bench-strtok.c (oldstrtok): Use rawmemchr.
	* benchtests/bench-timing.h: Define attribute_hidden.