clover: Allow overriding platform/device version numbers

clover: Allow overriding platform/device version numbers

Useful for testing API, builtin library, and device completeness of
not-yet-supported versions.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
(v3) Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Jan Vesely <jan.vesely@rutgers.edu>
v4: Remove redundant std::string wrapper around debug_get_option calls
v3: mark CL version overrides as static and const
v2: Make version_string in platform const in case