[swift_build_support] Add support for building benchmarks on linux-x86_64.

I put in a small little kludge to add support for this. The better solution is
to refactor the supports_benchmark property from Platform to Target. We
already have a similar abstraction problem with armv7 target on Darwin, where we
explicitly turn off benchmarking.

*NOTE* Building the benchmarks for linux still requires more cmake work. I am
making some changes in a subsequent commit series to do this. If you try to
compile the benchmarks, you will hit an error where ninja says that it doesn't
know about the swift-benchmark-linux-x86_64 target.

Author: gottesmm

utils/swift_build_support/swift_build_support/targets.py

@@ -119,6 +119,11 @@ class StdlibDeploymentTarget(object):
         "powerpc64",
         "powerpc64le",
         "s390x"])
+    # We support build/test benchmarks only on Linux x86_64.
+    #
+    # TODO: If we support any more platforms, we should consider refactoring the
+    # supports_benchmark property onto the target from the platform.
+    Linux.x86_64.supports_benchmark = True
 
     FreeBSD = Platform("freebsd", archs=["x86_64"])