[SPARK-14656][CORE] Fix Benchmark.getPorcessorName() always return "Unknown processor" on Linux

## What changes were proposed in this pull request?
This PR returns correct processor name in ```/proc/cpuinfo``` on Linux from  ```Benchmark.getPorcessorName()```. Now, this return ```Unknown processor```.
Since ```Utils.executeAndGetOutput(Seq("which", "grep"))``` return ```/bin/grep\n```, it is failed to execute ```/bin/grep\n```. This PR strips ```\n``` at the end of the line of a result of ```Utils.executeAndGetOutput()```

Before applying this PR
````
Java HotSpot(TM) 64-Bit Server VM 1.8.0_66-b17 on Linux 2.6.32-504.el6.x86_64
Unknown processor
back-to-back filter:                Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
-------------------------------------------------------------------------------------------
Dataset                                   472 /  503         21.2          47.2       1.0X
DataFrame                                  51 /   58        198.0           5.1       9.3X
RDD                                       189 /  211         52.8          18.9       2.5X
````

After applying this PR
```
Java HotSpot(TM) 64-Bit Server VM 1.8.0_66-b17 on Linux 2.6.32-504.el6.x86_64
Intel(R) Xeon(R) CPU E5-2667 v2  3.30GHz
back-to-back filter:                Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
-------------------------------------------------------------------------------------------
Dataset                                   490 /  502         20.4          49.0       1.0X
DataFrame                                  55 /   61        183.4           5.5       9.0X
RDD                                       210 /  237         47.7          21.0       2.3X
```

## How was this patch tested?
Run Benchmark programs on Linux by hand

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #12411 from kiszk/SPARK-14656.

core/src/main/scala/org/apache/spark/util/Benchmark.scala

@@ -96,9 +96,9 @@ private[spark] object Benchmark {
       Utils.executeAndGetOutput(Seq("/usr/sbin/sysctl", "-n", "machdep.cpu.brand_string"))
     } else if (SystemUtils.IS_OS_LINUX) {
       Try {
-        val grepPath = Utils.executeAndGetOutput(Seq("which", "grep"))
+        val grepPath = Utils.executeAndGetOutput(Seq("which", "grep")).stripLineEnd
         Utils.executeAndGetOutput(Seq(grepPath, "-m", "1", "model name", "/proc/cpuinfo"))
-        .replaceFirst("model name[\\s*]:[\\s*]", "")
+        .stripLineEnd.replaceFirst("model name[\\s*]:[\\s*]", "")
       }.getOrElse("Unknown processor")
     } else {
       System.getenv("PROCESSOR_IDENTIFIER")