spirv: Add an execution environment to the options

spirv: Add an execution environment to the options

Also updates gl_spirv to pick the right one.  At the moment nothing
uses it, but upcoming functionality part of ARB_gl_spirv will use it,
and we also later can be more assertful when handling certain features
for each of the execution environments.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Acked-by: Karol Herbst <kherbst@redhat.com>