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.