ExternalProject: Restore default log dir with custom stamp dir

In commit b6f6cac378 (ExternalProject: add LOG_DIR option that allows
overriding of log location, 2018-10-12, v3.14.0-rc1~515^2~1) the log
directory got its own option.  The intention was to fall back to the
stamp directory by default.  However, the implementation actually only
falls back to the same default as the stamp directory and does not
consider a custom stamp dir.

Update the default log dir computation to fall back to whatever is the
final selection for the stamp dir.

Fixes: #19000

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