benchtests: Support for cross-building benchmarks

benchtests: Support for cross-building benchmarks

This patch adds full support for cross-building benchmarks.  Some
benchmarks like those that need locales to be generated cannot be
built and are hence skipped for cross builds.

Tested by cross building for aarch64 on x86_64 and then running the
generated benchmark on aarch64.

	* benchtests/Makefile (wcsmbs-benchset): Include only for
	native builds and runs.
	(LOCALES): Likewise.
	(bench-build): Build timing-type here instead of the bench
	target.  Generate locale only for native builds.
	* benchtests/README: Add note for cross-building.