[gdb/testsuite] Fix re-read FAILs with -readnow

[gdb/testsuite] Fix re-read FAILs with -readnow

When running the testsuite with target board readnow, we run into:
...
FAIL: gdb.ada/exec_changed.exp: start second
FAIL: gdb.ada/exec_changed.exp: start just first
FAIL: gdb.base/reread.exp: opts= "" "" : run to foo() second time
FAIL: gdb.base/reread.exp: opts= "" "" : second pass: run to foo() second time
FAIL: gdb.base/reread.exp: opts= "-fPIE" "ldflags=-pie" : \
  run to foo() second time
FAIL: gdb.base/reread.exp: opts= "-fPIE" "ldflags=-pie" : second pass: \
  run to foo() second time
...

These are FAILs due to PR26800.

Mark these as KFAILs.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-28  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/exec_changed.exp: Add KFAILs for -readnow.
	* gdb.base/reread.exp: Same.