gdb: include jit_code_entry::symfile_addr value in names of objfiles created by jit reader API

This commit includes the JIT object's symfile address in the names of
objfiles created by JIT reader API (e.g., << JIT compiled code at
0x7ffd8a0c77a0 >>).  This allows one to at least differentiate one from
another.

The address is the one that the debugged program has put in
jit_code_entry::symfile_addr, and that the JIT reader's read function
receives.  As we can see in gdb.base/jit-reader-host.c and
gdb.base/jit-reader.c, that may not be the actual value of where the
JIT-ed code is.  But it is a value chosen by the author of the JIT
engine and the JIT reader, so including this value in the objfile name
may help them correlate the JIT objfiles created by with their logs /
data structures.

To access this field, we need to pass down a reference to the
jit_code_entry.  So make jit_dbg_reader_data a structure (instead of an
alias for a CORE_ADDR) that includes the address of the code entry in
the inferior's address space (the previous meaning of
jit_dbg_reader_data) plus a reference to the jit_code_entry as read into
GDB's address space.  And while at it, pass down the gdbarch, so that we
don't have to call target_gdbarch.

Co-Authored-By: Jan Vrany <jan.vrany@labware.com>
Change-Id: Ib26c4d1bd8de503d651aff89ad2e500cb312afa5