base: Switch virtual/cross-XXX to be under recipe specific providers

base: Switch virtual/cross-XXX to be under recipe specific providers

Currently, providers are set on a global config basis. This change allows
for a select set of providers to be configured using BB_RECIPE_VIRTUAL_PROVIDERS
on a per recipe basis. This would allow for the selection of virtual/cross-cc
as gcc or clang for example.

The PROVIDERS are removed from the recipes so that if a version of the
dependency accidentally slips through, the build will fail and the user
can correct the issue.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>