[gdb/testsuite] Fix gdb.dwarf2/dw2-stack-boundary.exp with -readnow

[gdb/testsuite] Fix gdb.dwarf2/dw2-stack-boundary.exp with -readnow

When running test-case gdb.dwarf2/dw2-stack-boundary.exp with target board
readnow, we run into:
...
FAIL: gdb.dwarf2/dw2-stack-boundary.exp: check partial symtab errors
...

The cause for the FAIL is that these complaints are not there:
...
During symbol reading: location description stack underflow^M
During symbol reading: location description stack overflow^M
...

Fix this by KFAILing the complaints for -readnow.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

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

	* gdb.dwarf2/dw2-stack-boundary.exp: KFAILing the complaints for
	-readnow.