Enable mold linker by default

Enables linking with mold by default if it's available on the system.
Mold is around 2x as fast as lld on my system.
I've been using for it a short while and it supports all the features we
need and I haven't encountered any issues so far.

Author: karolzwolak

IGC/CMakeLists.txt

@@ -1737,7 +1737,7 @@ if(UNIX)
   endfunction()
 
   # List of linkers to auto-detect when user didn't specify one
-  set(_default_linkers lld ld)
+  set(_default_linkers mold lld ld)
 
   if (USE_LINKER STREQUAL "")
       message(STATUS "No linker specified — probing system")