Test failure on darwin x86-64 in DWARF2 mode

DWARF2 does not have a code for the Rust language, and is the default on some darwin systems. This causes one failure in our testsuite.

            │17:59:56       iains |   === rust Summary ===                                              
            │17:59:56       iains | # of expected passes  4989                                             
            │17:59:57       iains | # of unexpected failures 1                                         
            │17:59:58       iains | # of expected failures  39                                             
            │18:00:12 cohenarthur | uh                                                                    
            │18:00:19 cohenarthur | do you have the failure's name?                                     
            │18:00:53       iains | te failure is from DW_AT_encoding which is likely related to the default  
            │                     | for Darwin of DWARF2 (I am in the process of trying to fix that for systems
            │                     | > macOS 10.10 .. but ...                                                    
            │18:01:14       iains | I guess DWARF2 does not have a value for rust  
            │18:01:38       iains | rust/debug/chartype.rs scan-assembler 0x10[ \t][^\n\r]* DW_AT_encoding     
            │18:01:57       iains | but that’s not too bad for day #1                 
            │18:04:33       iains | you could add -gdwarf-4 for that test .. I guess (it should still pass on   
            │                     | darwin then, since darwin only sets dwarf2 as the default, the user is free 
            │                     | to override)          

Start with rust/debug/chartype.rs and its scan-assembler check for DW_AT_encoding. Run that test on Darwin x86-64 in DWARF2 mode, then verify the test passes with the appropriate DWARF setting. Done means the Rust testsuite no longer reports this failure on affected Darwin systems.