cmake: fix unity with Windows Unicode + TrackMemory

cmake: fix unity with Windows Unicode + TrackMemory

Found the root cause of the startup crash in unity builds with Unicode
and TrackMemory enabled at the same time.

We must make sure that the `memdebug.h` header doesn't apply to
`lib/curl_multibyte.c` (as even noted in a comment there.) In unity
builds all headers apply to all sources, including `curl_multibyte.c`.
This probably resulted in an infinite loop on startup.

Exclude this source from unity compilation with TrackMemory enabled,
in both libcurl and curl tool. Enable unity mode for a debug Unicode
CI job to keep it tested. Also delete the earlier workaround that
fully disabled unity for affected builds.

Follow-up to d82b080f6374433ce7c98241329189ad2d3976f8 #12005
Follow-up to 3f8fc25720900b14b7432f4bd93407ca15311719 #11095

Closes #11928