external-sourcery-toolchain: add libgcov

This is needed to avoid the real libgcc being pulled in by
packagegroup-core-standalone-sdk-target when building an SDK.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
tcmode: don't set SDKGCCVERSION

There's no need for the crosssdk and nativesdk gcc bits to be the same version
as the external toolchain.

JIRA: SB-3919

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #53 from kergoth/sb-3919

Fix a few issues with populate_sdk