Ensure 'exec-file has changed' check has priority over 'exec-file-mismatch' check

Ensure 'exec-file has changed' check has priority over 'exec-file-mismatch' check

Following the implementation of exec-file-mismatch based on build-id,
an attach to a process that runs a modified exec-file was triggering
the exec-file-mismatch handling, giving a warning such as:
  warning: Mismatch between current exec-file /bd/home/philippe/gdb/git/build_termours/gdb/testsuite/outputs/gdb.base/attach/attach
  and automatically determined exec-file /bd/home/philippe/gdb/git/build_termours/gdb/testsuite/outputs/gdb.base/attach/attach
  exec-file-mismatch handling is currently "ask"
as the build-ids differ when an exec-file is recompiled.

This patch ensures that the exec-file-mismatch check is done with an up to date
build-id.  With this, exec-file-mismatch check will only trigger when the
PID file really differs from the (build-id refreshed) current exec-file.
Note that the additional check does not (yet) reload the symbols if
the exec-file is changed: this reload will happen later if needed.

gdb/ChangeLog
2020-06-21  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* exec.c (validate_exec_file): Ensure the build-id is up to
	date by calling reopen_exec_file (that checks file timestamp
	to decide to re-read the file).

gdb/testsuite/ChangeLog

2020-06-21  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.base/attach.exp: Test priority of 'exec-file' changed
	over 'exec-file-mismatch'.
	* gdb.base/attach.c: Mark should_exit volatile.
	* gdb.base/attach2.c: Likewise.  Add a comment explaining
	why the sleep cannot be big.
	* gdb.base/attach3.c: New file.