[gdb/testsuite] Fix have_mpx test

When testing on openSUSE Leap 15.4 I ran into this FAIL:
...
FAIL: gdb.arch/i386-mpx-map.exp: NULL address of the pointer
...
and likewise for all the other mpx tests.

The problem is that have_mpx is supposed to return 0, but it doesn't because
it tries to match this output:
...
builtin_spawn -ignore SIGHUP temp/20294/have_mpx-2-20294.x^M
No MPX support^M
No MPX support^M
...
using:
...
                   && ![string equal $output "No MPX support\r\n"]]
...

Fix this by matching using a regexp instead.

Tested on x86_64-linux.

gdb/testsuite/lib/gdb.exp

@@ -8479,7 +8479,7 @@ gdb_caching_proc have_mpx {
     set status [lindex $result 0]
     set output [lindex $result 1]
     set status [expr ($status == 0) \
-		    && ![string equal $output "No MPX support\r\n"]]
+		    && ![regexp "^No MPX support\r\n" $output]]
 
     remote_file build delete $obj