gdb/testsuite: Ensure constant test name in gdb.base/break-interp.exp

When running the testsuite, I have lines similar to the following in the
gdb.sum file:

~~~
PASS: gdb.base/break-interp.exp: ldprelink=NO: ldsepdebug=NO: first backtrace: p /x 0x7f283d2f0fd1
...
PASS: gdb.base/break-interp.exp: ldprelink=NO: ldsepdebug=NO: binprelink=NO: binsepdebug=NO: binpie=NO: INNER: first backtrace: p /x 0x7f00de0317a5
...
~~~

The address part of the command might change between execution of the
test, which adds noise to a diff between two .sum files.

This patch changes to test name to "p /x $pc" in order to have constant
test name.

Tested on x86_64-Linux.

Change-Id: I973c1237a084dd6d424276443cbf0920533c9a21