[gdb/testsuite] Handle missing xz in gdb.base/gnu-debugdata.exp

[gdb/testsuite] Handle missing xz in gdb.base/gnu-debugdata.exp

When running test-case gdb.base/gnu-debugdata.exp on SLE-11, I run into:
...
FAIL: gdb.base/gnu-debugdata.exp: xz
...

The fact that xz is not installed does not mean there's a fail, merely that
the test is unsupported.

Fix this by detecting the "spawn failed" reply in run_on_host and issuing
UNSUPPORTED instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-14  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26963
	* lib/gdb.exp (run_on_host): Declare test unsupported if spawn fails.