gdb: testsuite: Test whether PC register is expedited in gdb.server/server-run.exp

gdb: testsuite: Test whether PC register is expedited in gdb.server/server-run.exp

One thing GDB always does when the inferior stops is finding out where
it's stopped at, by way of querying the value of the program counter
register.

To save a packet round trip, the remote target can send the PC
value (often alongside other frequently consulted registers such as the
stack pointer) in the stop reply packet as an "expedited register".

Test that this is actually done for the targets where gdbserver is
supposed to.

Extend the "maintenance print remote-registers" command output with an
"Expedited" column which says "yes" if the register was seen by GDB in
the last stop reply packet it received, and is left blank otherwise.

Tested for regressions on aarch64-linux-gnu native-extended-remote.

The testcase was tested on aarch64-linux-gnu, i686-linux-gnu and
x86_64-linux-gnu native-remote and native-extended-remote targets.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>