runtime: TestGdbPython flaky on linux-mipsle builder

Examples: https://build.golang.org/log/e59511fa905a9939c7b85a9f6cefb99140c8915f https://build.golang.org/log/dc9b43b3eb5e0029089cf9ac53b37f6b8b0e7847

Detail:

##### GOMAXPROCS=2 runtime -cpu=1,2,4 -quick
--- FAIL: TestGdbPython (3.10s)
    runtime-gdb_test.go:61: gdb version 7.7
    runtime-gdb_test.go:195: gdb output: Loading Go Runtime support.
        Loaded  Script                                                                 
        Yes     /data/mipsle/go/src/runtime/runtime-gdb.py                             
        Breakpoint 1 at 0x9b508: file /data/mipsle/go/src/fmt/print.go, line 264.
        Failed to read a valid object file image from memory.
        
        Breakpoint 1, fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN info goroutines
        * 1 running  runtime.systemstack_switch
          2 runnable runtime.forcegchelper
          3 waiting  runtime.gopark
          4 runnable runtime.runfinq
        END
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        13		slicevar = append(slicevar, mapvar["abc"])
        BEGIN print mapvar
        $1 = map[string]string = {["abc"] = "def", ["ghi"] = "jkl"}
        END
        BEGIN print strvar
        $2 = "abc"
        END
        BEGIN info locals
        mapvar = map[string]string = {["abc"] = "def", ["ghi"] = "jkl"}
        slicevar =  []string = {"def"}
        strvar = "abc"
        END
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN goroutine 1 bt
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        BEGIN goroutine 2 bt
        #0  runtime.forcegchelper () at /data/mipsle/go/src/runtime/proc.go:243
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        Breakpoint 2 at 0xa2514: file /tmp/go-build482556707/main.go, line 18.
        
        Program received signal SIGTRAP, Trace/breakpoint trap.
        fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN goroutine 1 bt at the end
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        
    runtime-gdb_test.go:264: goroutine 1 bt at the end failed: #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
FAIL
FAIL	runtime	208.804s