Don't remove only libpthread on latest libc

Author: BtbN

images/base-linuxmips64/Dockerfile

@@ -29,7 +29,6 @@ RUN \
         -or -name 'libm.a' \
         -or -name 'libmvec.a' \
         -or -name 'librt.a' \
-        -or -name 'libpthread.a' \
         \) -delete && \
     mkdir /opt/ffbuild
 

images/base-linuxppc64/Dockerfile

@@ -29,7 +29,6 @@ RUN \
         -or -name 'libm.a' \
         -or -name 'libmvec.a' \
         -or -name 'librt.a' \
-        -or -name 'libpthread.a' \
         \) -delete && \
     mkdir /opt/ffbuild