Avoid timeouts in gdb.compile

Avoid timeouts in gdb.compile

PR compile/29541 points out that some of the C++ tests in gdb.compile
will time out when the glibc debuginfo is installed.  This was
interfering with my hacking on gdb by making test runs extremely long,
so I looked into it.

Internally the bug seems to be that gdb tries to convert multiple
symbols named "var" via the compiler interface; one such symbol (I
didn't track it down too far) causes the C++ compiler plugin to crash.

Unfortunately, the crash is reported as a timeout, as the gdb side of
the plugin simply hangs.  This seems like a bug in the plugin RPC
mechanism and, worse, apparently when I wrote this stuff I didn't
really consider error reporting very much at all, so gdb can't really
detect failures in the first place.

Anyway... this patch works around the timeout by compiling a simple
test that should provoke this bug, and then using "untested" if it
notices a GCC crash.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29541