gdb/testsuite: fix gdb.base/break-idempotent.exp on ppc

gdb/testsuite: fix gdb.base/break-idempotent.exp on ppc

When running the gdb.base/break-idempotent.exp test on ppc, I was
seeing some test failures (or rather errors), that looked like this:

  (gdb) watch local
  Hardware watchpoint 2: local

  has_hw_wp_support: Hardware watchpoint detected
  ERROR: no fileid for gcc2-power8
  ERROR: Couldn't send delete breakpoints to GDB.
  ERROR OCCURED: can't read "gdb_spawn_id": no such variable
      while executing
  "expect {
  -i 1000 -timeout 100
          -re ".*A problem internal to GDB has been detected" {
              fail "$message (GDB internal error)"
              gdb_internal_erro..."
      ("uplevel" body line 1)
      invoked from within

What happens is that in break-idempotent.exp we basically do this:

    if {[prepare_for_testing "failed to prepare" $binfile $srcfile $opts]} {
        continue
    }

    # ....

    if {![skip_hw_watchpoint_tests]} {
        test_break $always_inserted "watch"
    }

The problem with this is that skip_hw_watchpoint_tests, includes this:

    if { [istarget "i?86-*-*"]
	 || [istarget "x86_64-*-*"]
	 || [istarget "ia64-*-*"]
	 || [istarget "arm*-*-*"]
	 || [istarget "aarch64*-*-*"]
	 || ([istarget "powerpc*-*-linux*"] && [has_hw_wp_support])
	 || [istarget "s390*-*-*"] } {
	return 0
    }

For powerpc only we call has_hw_wp_support.  This is a caching proc
that runs a test within GDB to detect if we have hardware watchpoint
support or not.

Unfortunately, to run this test we restart GDB, and when the test has
completed, we exit GDB.  This means that in break-idempotent.exp, when
we call skip_hw_watchpoint_tests for the first time on powerpc, GDB
will unexpectedly be exited.  When we later call delete_breakpoints we
see the errors I reported above.

The fix is to call skip_hw_watchpoint_tests early, before we start GDB
as part of the break-idempotent.exp script, and store the result in a
variable, we can then check this variable in the script as needed.

After this change break-idempotent.exp runs fine on powerpc.

Co-authored-by: Andrew Burgess <aburgess@redhat.com>