gdb: include the base address in in-memory bfd filenames

gdb: include the base address in in-memory bfd filenames

The struct target_buffer (in gdb_bfd.c) is used to hold information
about an in-memory BFD object created by GDB.  For now this mechanism
is used by GDB when loading information about JIT symfiles.

This commit updates target_buffer (in gdb_bfd.c) to be more C++ like,
and, at the same time, adds the base address of the symfile into the
BFD filename.

Right now, every in-memory BFD is given the filename "<in-memory>".
This filename is visible in things like 'maint info symtabs' and
'maint info line-table'.  If there are multiple in-memory BFD objects
then it can be hard to match keep track if which BFD is which.  This
commit changes the name to be "<in-memory@ADDRESS>" where ADDRESS is
replaced with the base address for where the in-memory symbol file was
read from.

As an example of how this is useful, here's the output of 'maint info
jit' showing a single loaded JIT symfile:

  (gdb) maintenance info jit
  jit_code_entry address symfile address    symfile size
  0x00000000004056b0     0x0000000007000000 17320

And here's part of the output from 'maint info symtabs':

  (gdb) maintenance info symtabs
  ...snip...
  { objfile <in-memory@0x7000000> ((struct objfile *) 0x5258250)
    { ((struct compunit_symtab *) 0x4f0afb0)
      debugformat DWARF 4
      producer GNU C17 9.3.1 20200408 (Red Hat 9.3.1-2) -mtune=generic -march=x86-64 -g -fno-stack-protector -fpic
      name jit-elf-solib.c
      dirname /tmp/binutils-gdb/build/gdb/testsuite
      blockvector ((struct blockvector *) 0x5477850)
      user ((struct compunit_symtab *) (null))
  	{ symtab /tmp/binutils-gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/jit-elf-solib.c ((struct symtab *) 0x4f0b030)
  	  fullname (null)
  	  linetable ((struct linetable *) 0x5477880)
  	}
    }
  }

I've added a new test that checks the new in-memory file names are
generated correctly, and also checks that the in-memory JIT files can
be dumped back out using 'dump binary memory'.