gdb: Add test for some error cases of @entry usage
    
    Adds a test that using @entry for a non-parameter, or for an unknown
    symbol, both give the expected error.  This error message was
    previously untested.
    
    gdb/testsuite/ChangeLog:
    
    	* gdb.arch/amd64-entry-value.exp: Test using @entry on a
    	non-parameter, and on an unknown symbol.