devtools: unify cross-compilation tests

devtools: unify cross-compilation tests

Reduce the number of Arm builds from 3 to 1:
only generic armv8 with GCC.
The specific PPC builds on Ubuntu are skipped.

The build directories for PPC and RISC-V
are also renamed for consistency:
	- build-arm64-generic-gcc
	- build-ppc64-power8-gcc
	- build-riscv64-generic-gcc

The cross file is always saved in variable "f" for readability.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>