Improve build-rpm.sh script
Unset DESTDIR in pkg.sh and edit v8 CXXFLAGS
Remove a reference to the old support.mk
Always use --host flag when compiling libunwind
Use the correct architecture flag when building RPM
Fix v8 build error on centos
Merge branch atnnn/2183 into next

Closes #2183
Closes #2176
Closes #1491

Review 1380 by @mlucy