Making platform 007 (gcc 7) default in build_detect_platform.sh (#5947)

Summary:
Making platform 007 (gcc 7) default in build_detect_platform.sh.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5947

Differential Revision: D18038837

Pulled By: vjnadimpalli

fbshipit-source-id: 9ac2ddaa93bf328a416faec028970e039886378e

build_tools/build_detect_platform

@@ -56,10 +56,10 @@ if [ -z "$ROCKSDB_NO_FBCODE" -a -d /mnt/gvfs/third-party ]; then
     if [ -n "$ROCKSDB_FBCODE_BUILD_WITH_481" ]; then
       # we need this to build with MySQL. Don't use for other purposes.
       source "$PWD/build_tools/fbcode_config4.8.1.sh"
-    elif [ -n "$ROCKSDB_FBCODE_BUILD_WITH_PLATFORM007" ]; then
-      source "$PWD/build_tools/fbcode_config_platform007.sh"
-    else
+    elif [ -n "$ROCKSDB_FBCODE_BUILD_WITH_5xx" ]; then
       source "$PWD/build_tools/fbcode_config.sh"
+    else
+      source "$PWD/build_tools/fbcode_config_platform007.sh"
     fi
 fi