Regression test build binaries with PORTABLE=1

Summary:
We hit "Illegal instruction" error in regression test with "shlx" instruction. Setting PORTABLE=1 to resolve it.
Closes https://github.com/facebook/rocksdb/pull/3165

Differential Revision: D6321972

Pulled By: yiwu-arbug

fbshipit-source-id: cc9fe0dbd4698d1b66a750a0b062f66899862719

tools/regression_test.sh

@@ -376,7 +376,7 @@ function build_db_bench_and_ldb {
   make clean
   exit_on_error $?
 
-  DEBUG_LEVEL=0 make db_bench ldb -j32
+  DEBUG_LEVEL=0 PORTABLE=1 make db_bench ldb -j32
   exit_on_error $?
 }