The quick-and-dirty answer is to add exit 0 at the end of the %install script which disables various helpers that are appended to the end of the %install build scriptlet when run.
The more specific answer is to examine the helper scriptlets (which strip shared libraries of debugging symbols) using rpm --showrc on your system and modify what helpers are being run.
The two most important macros that need to be configured to diable debug symbol stripping are:
# Define per-arch and per-os defaults. Normally overridden by per-target macros.
%__arch_install_post %{nil}
%__os_install_post %{___build_post}
Which are usually re-defined in the redhat-rpm-config package.