gdb: extend the information printed by 'maint info jit'

This commit updates the output of 'maint info jit' to print not just
the jit_code_entry address, but also the symfile address, and the
symfile size.

The new information could be obtained by looking into target memory at
the contents of the jit_code_entry, but, by storing this information
within gdb at the time the jit object is loaded, it is now possible to
check if the jit_code_entry has been modified in target memory behind
gdb's back.

Additionally, the symfile address is the same address that is now used
in the objfile names after commit 4a620b7e.

One test that relies on the output of 'maint info jit' was updated to
allow for the new output format.