gdb: add a symbol* argument to get_return_value

gdb: add a symbol* argument to get_return_value

Add an argument to the get_return_value function to indicate the symbol
of the function the debuggee is returning from.  This will be used by
the following patch.

Since the function return type can be deduced from the symbol remove the
value_type argument which becomes redundant.

No user visible change after this patch.

Tested on x86_64-linux.

Change-Id: Idf1279f1f7199f5022738a6679e0fa63fbd22edc
Co-authored-by: Simon Marchi <simon.marchi@polymtl.ca>