runtime: TestGdb* triggers a GDB bug on the linux-mips64-rtrk builder

[bcmills]

Several failures like this one seem to indicate that a broken gdb is installed on the linux-mips64-rtrk builder. It should either be upgraded to a fixed version, or removed (or removed from $PATH) so that the tests will skip.

--- FAIL: TestGdbPanic (3.84s)
    runtime-gdb_test.go:65: gdb version 8.2
    runtime-gdb_test.go:599: could not find '(#.* .* in )?main\.crash' in backtrace
    runtime-gdb_test.go:600: gdb output:
        Loading Go Runtime support.
        /build/gdb-XNneqN/gdb-8.2.1/gdb/mips-tdep.c:776: internal-error: bad register size
        A problem internal to GDB has been detected,
        further debugging may prove unreliable.
        Quit this debugging session? (y or n) [answered Y; input not from terminal]
        
        This is a bug, please report it.  For instructions, see:
        <http://www.gnu.org/software/gdb/bugs/>.
        
        /build/gdb-XNneqN/gdb-8.2.1/gdb/mips-tdep.c:776: internal-error: bad register size
        A problem internal to GDB has been detected,
        further debugging may prove unreliable.
        Create a core file of GDB? (y or n) [answered Y; input not from terminal]

Same for TestGdbBacktrace, TestGdbConst, TestGdbAutotmpTypes, TestGdbPython.

2019-11-08T23:22:06-11da2b2/linux-mips64-rtrk
2019-11-08T21:05:17-9ee6ba0/linux-mips64-rtrk

CC @bogojevic @milanknezevic

[cherrymui]

The builder has 32-bit user space. I don't think 32-bit gdb can handle 64-bit programs.