BareMetal: Add support for Keil C251 toolchain

This patch adds support for the C251 toolchain, provided by KEIL:

 * http://www.keil.com/c251/

Now the auto-detected C251 compiler will be displayed with an
appropriate ABI on the "Options->Kits->Compilers" page.

Change-Id: I40cf2b82117420b2eddb09cad902c9f264bac062
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/abi.cpp

@@ -705,6 +705,8 @@ QString Abi::toString(const Architecture &a)
         return QLatin1String("x86");
     case Mcs51Architecture:
         return QLatin1String("mcs51");
+    case Mcs251Architecture:
+        return QLatin1String("mcs251");
     case MipsArchitecture:
         return QLatin1String("mips");
     case PowerPCArchitecture:
@@ -849,6 +851,8 @@ Abi::Architecture Abi::architectureFromString(const QStringRef &a)
         return X86Architecture;
     if (a == "mcs51")
         return Mcs51Architecture;
+    if (a == "mcs251")
+        return Mcs251Architecture;
     if (a == "mips")
         return MipsArchitecture;
     if (a == "ppc")