gdb: print compunit_symtab name in "maint info symtabs"

gdb: print compunit_symtab name in "maint info symtabs"

I think it would make sense to print a compunit_symtab's name in "maint
info symtabs".  If you are looking for a given CU in the list, that's
probably the field you will be looking at.  As the doc of
compunit_symtab::name says, it is not meant to be a reliable file name,
it is for debugging purposes (and "maint info symtabs" exists for
debugging purposes).

Sample output with the new field:

    (gdb) maintenance info symtabs
    { objfile /home/simark/build/binutils-gdb-one-target/gdb/a.out ((struct objfile *) 0x613000005d00)
      { ((struct compunit_symtab *) 0x621000131630)
        debugformat DWARF 5
        producer GNU C17 11.2.0 -mtune=generic -march=x86-64 -g3 -O0
        name test.c
        dirname /home/simark/build/binutils-gdb-one-target/gdb
        blockvector ((struct blockvector *) 0x621000131d10)
        user ((struct compunit_symtab *) (null))
            { symtab test.c ((struct symtab *) 0x6210001316b0)
              fullname (null)
              linetable ((struct linetable *) 0x621000131d40)
            }
            { symtab /home/simark/build/binutils-gdb-one-target/gdb/test.h ((struct symtab *) 0x6210001316e0)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
            { symtab /usr/include/stdc-predef.h ((struct symtab *) 0x621000131710)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }
      { ((struct compunit_symtab *) 0x6210001170a0)
        debugformat DWARF 5
        producer GNU C17 11.2.0 -mtune=generic -march=x86-64 -g3 -O0
        name foo.c
        dirname /home/simark/build/binutils-gdb-one-target/gdb
        blockvector ((struct blockvector *) 0x621000131580)
        user ((struct compunit_symtab *) (null))
            { symtab foo.c ((struct symtab *) 0x621000117120)
              fullname (null)
              linetable ((struct linetable *) 0x6210001315b0)
            }
            { symtab /usr/include/stdc-predef.h ((struct symtab *) 0x621000117150)
              fullname (null)
              linetable ((struct linetable *) 0x0)
            }
      }
    }

Change-Id: I17b87adfac2f6551cb5bda30d59f6c6882789211