[core] Fix LIB_CORE_NAME mismatch when soversion is enabled

On Linux, dl_iterate_phdr reports loaded libraries by their SONAME
(e.g. libCore.so.6.38), not their full filename (libCore.so.6.38.02).

Commit ff7e631 changed LIB_CORE_NAME from using the SOVERSION-based
name to TARGET_FILE_NAME, which gives the full versioned filename. When
soversion is enabled, this causes TROOT::GetSharedLibDir() to fail to
match the library name reported by the dynamic linker, returning an
empty path. This breaks module loading and causes cling to attempt
recompiling modules from source at runtime, which fails when external
headers (e.g. Vc) are not in cling's include path.

Fix by using TARGET_SONAME_FILE_NAME when soversion is enabled on Linux.
On macOS, _dyld_get_image_name returns the actual filename, so
TARGET_FILE_NAME remains correct there.

(cherry picked from commit 82162e3)

Author: chrisburr

core/base/CMakeLists.txt

@@ -212,7 +212,15 @@ if(ROOT_NEED_STDCXXFS)
 endif()
 
 # This code about the LIB_CORE_NAME define is important for TROOT::GetSharedLibDir()
-set(full_core_filename $<TARGET_FILE_NAME:Core>)
+# On Linux, dl_iterate_phdr reports loaded libraries by their SONAME
+# (e.g. libCore.so.6.38), not their full filename (libCore.so.6.38.02).
+# On macOS, _dyld_get_image_name returns the actual filename.
+# LIB_CORE_NAME must match what the dynamic linker reports.
+if(soversion AND NOT APPLE)
+  set(full_core_filename $<TARGET_SONAME_FILE_NAME:Core>)
+else()
+  set(full_core_filename $<TARGET_FILE_NAME:Core>)
+endif()
 
 target_compile_options(Core PRIVATE -DLIB_CORE_NAME=${full_core_filename})