[testsuite] Disable Ctrl-V use for mingw hosts.

[testsuite] Disable Ctrl-V use for mingw hosts.

On mingw host, we have seen two fails as below,

p int1dim[0]^V@2
Invalid character '^V' in expression.
(gdb) FAIL: gdb.base/printcmds.exp: p int1dim[0]@2
p int1dim[0]^V@2^V@3
Invalid character '^V' in expression.
(gdb) FAIL: gdb.base/printcmds.exp: p int1dim[0]@2@3

In the test, the comment says "# Send \026@ instead of just @ in case
the kill character is @".  Historically, kill character was @, and
Ctrl-V (\026) is to escape the next character.  However, we don't have
to do so on mingw.  This patch is to disable ctrl-v usage on mingw
hots.  With this patch applied, it becomes:

p int1dim[0]@2
$607 = {0, 1}
(gdb) PASS: gdb.base/printcmds.exp: p int1dim[0]@2
p int1dim[0]@2@3
$608 = {{0, 1}, {2, 3}, {4, 5}}

Note that this patch is picked from Pierre's submission,

  [RFC 6/6] Fix remaining failures in gdb.base/printcmds.exp for mingw hosts.
  https://www.sourceware.org/ml/gdb-patches/2013-09/msg00943.html

gdb/testsuite:

2014-04-08  Pierre Muller  <muller@sourceware.org>

	* gdb.base/printcmds.exp (test_artificial_arrays): Disable
	Ctrl-V use for mingw hosts.