build: use platform for generic and native builds

build: use platform for generic and native builds

The current meson option 'machine' should only specify the ISA, which is
not sufficient for Arm, where setting ISA implies other settings as well
(and is used in Arm configuration as such).
Use the existing 'platform' meson option to differentiate the type of
the build (native/generic) and set ISA accordingly, unless the user
chooses to override it with a new option, 'cpu_instruction_set'.
The 'machine' option set the ISA in x86 builds and set native/default
'build type' in aarch64 builds. These two new variables, 'platform' and
'cpu_instruction_set', now properly set both ISA and build type for all
architectures in a uniform manner.
The 'machine' option also doesn't describe very well what it sets. The
new option, 'cpu_instruction_set', is much more descriptive. Keep
'machine' for backwards compatibility.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>