When RELRO is set to Full, readline fails to build.  RELRO Partial has no issues.

/home/buildroot/project/buildroot# make
>>> readline 7.0 Building
PATH="/home/buildroot/project/buildroot/output/host/bin:/home/buildroot/project/buildroot/output/host/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"  /usr/bin/make -j16  -C /home/buildroot/project/buildroot/output/build/readline-7.0/
make[1]: Entering directory '/home/buildroot/project/buildroot/output/build/readline-7.0'
test -d shlib || mkdir shlib
( cd shlib ; /usr/bin/make -w -j --jobserver-fds=3,4 all )
make[2]: Entering directory '/home/buildroot/project/buildroot/output/build/readline-7.0/shlib'
rm -f libreadline.so.7.0
rm -f libhistory.so.7.0
/home/buildroot/project/buildroot/output/host/bin/powerpc64le-buildroot-linux-gnu-gcc -shared -Wl,-soname,libreadline.so.7.0 -pie -Wl,-z,now -Wl,-z,relro -Wl,-rpath,/usr/lib -Wl,-soname,`basename libreadline.so.7.0 .0` -o libreadline.so.7.0 readline.so vi_mode.so funmap.so keymaps.so parens.so search.so rltty.so complete.so bind.so isearch.so display.so signals.so util.so kill.so undo.so macro.so input.so callback.so terminal.so text.so nls.so misc.so history.so histexpand.so histfile.so histsearch.so shell.so mbutil.so tilde.so colors.so parse-colors.so xmalloc.so xfree.so compat.so -lncurses
/home/buildroot/project/buildroot/output/host/bin/powerpc64le-buildroot-linux-gnu-gcc -shared -Wl,-soname,libhistory.so.7.0 -pie -Wl,-z,now -Wl,-z,relro -Wl,-rpath,/usr/lib -Wl,-soname,`basename libhistory.so.7.0 .0` -o libhistory.so.7.0 history.so histexpand.so histfile.so histsearch.so shell.so mbutil.so xmalloc.so xfree.so -lncurses
/home/buildroot/project/buildroot/output/host/lib/gcc/powerpc64le-buildroot-linux-gnu/8.2.0/../../../../powerpc64le-buildroot-linux-gnu/bin/ld: /home/buildroot/project/buildroot/output/host/powerpc64le-buildroot-linux-gnu/sysroot/usr/lib/../lib64/Scrt1.o:(.data.rel.ro.local+0x8): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:180: recipe for target 'libhistory.so.7.0' failed
make[2]: *** [libhistory.so.7.0] Error 1
make[2]: *** Waiting for unfinished jobs....
/home/buildroot/project/buildroot/output/host/lib/gcc/powerpc64le-buildroot-linux-gnu/8.2.0/../../../../powerpc64le-buildroot-linux-gnu/bin/ld: /home/buildroot/project/buildroot/output/host/powerpc64le-buildroot-linux-gnu/sysroot/usr/lib/../lib64/Scrt1.o:(.data.rel.ro.local+0x8): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:176: recipe for target 'libreadline.so.7.0' failed
make[2]: *** [libreadline.so.7.0] Error 1
make[2]: Leaving directory '/home/buildroot/project/buildroot/output/build/readline-7.0/shlib'
Makefile:209: recipe for target 'shared' failed
make[1]: *** [shared] Error 2
make[1]: Leaving directory '/home/buildroot/project/buildroot/output/build/readline-7.0'
package/pkg-generic.mk:229: recipe for target '/home/buildroot/project/buildroot/output/build/readline-7.0/.stamp_built' failed
make: *** [/home/buildroot/project/buildroot/output/build/readline-7.0/.stamp_built] Error 2