[BOLT] Introduce lite processing mode without relocations

Summary:
When optimizing a binary without relocations, we can skip processing
functions without profile (cold functions). By skipping processing of
cold functions, we reduce the processing time and memory. We call
such mode a lite mode, and it is enabled by default.

Some processing is still done for functions without profile even in lite
mode. scanExternalRefs() function is used to detect secondary entry
points to functions that are not marked in the symbol table.

Note that the no-relocation requirement is a temporary limitation
of the lite mode.

(cherry picked from FBD21366567)