devtools: shrink cross-compilation test definition

Each cross-compilation case needs to define the target compiler
and the meson cross file.
Given the compiler is already defined in the cross file,
the latter is enough.

The function "build" is changed to accept a cross file alternatively
to the compiler name. In the case of a file (detected if readable),
the compiler is extracted with sed and tr, and the option --cross-file
is automatically added.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Christensen <drc@linux.vnet.ibm.com>