Fix embedded coreclr detection in corhost.cpp (#80294)

Fix embedded coreclr detection in corhost.cpp (#80294)

The `CORECLR_EMBEDDED` define that is used in corhost.cpp to detect
whether the current host has coreclr and some other native libraries
embedded or not doesn't work. The reason is that corhost.cpp is not
compiled separately for the cases of embedded and non-embedded coreclr.

The proper way is to check the `g_coreclr_embedded` global variable
that is defined in the ceemain.cpp, which is compiled separately for
those two cases.

While we could also make the corhost.cpp build twice, it would be
a waste of time.