gdb/varobj: Reset varobj after relocations have been computed

gdb/varobj: Reset varobj after relocations have been computed

[This patch is a followup to the discussion in
https://sourceware.org/pipermail/gdb-patches/2022-August/191188.html]

PR/29426 shows failures when running the gdb.mi/mi-var-invalidate-shlib
test when using a compiler which does not produce a PIE executable by
default.

In the testcase, a varobj is created to track a global variable, and
then the main binary is reloaded in GDB (using the file command).

During the load of the new binary, GDB tries to recreate the varobj to
track the global in the new binary (varobj_invalidate_iter).  At this
point, the old process is still in flight.  So when we try to access to
the value of the global, in a PIE executable we only have access to the
unrelocated address (the objfile's text_section_offset () is 0).  As a
consequence down the line read_value_memory fails to read the unrelated
address, so cannot evaluate the value of the global.  Note that the
expression used to access to the global’s value is valid, so the varobj
can be created.  When using a non PIE executable, the address of the
global GDB knows about at this point does not need relocation, so
read_value_memory can access the (old binary’s) value.

So at this point, in the case of a non-PIE executable the value field is
set, while it is cleared in the case of PIE executable.  Later when the
test issues a "-var-update global_var", the command sees no change in
the case of the non-PIE executable, while in the case of the PIE
executable install_new_value sees that value changes, leading to a
different output.

This patch makes sure that, as we do for breakpoints, we wait until
relocation has happened before we try to recreate varobjs.  This way we
have a consistent behavior between PIE and non-PIE binaries.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29426
Co-authored-by: Lancelot SIX <lancelot.six@amd.com>