gitian: remove execstack workaround for ricv64 & powerpc64le

When building with g++-10 (or 8) on Focal, binaries are being produced
with noexecstack by default, so we can remove the workaround of
explicitly passing "-Wl,-z,noexecstack" for risvc46 and powerpc64le.

When building for powerpc64 this is still required.

contrib/gitian-descriptors/gitian-linux.yml

@@ -132,7 +132,7 @@ script: |
   # Extract the git archive into a dir for each host and build
   for i in ${HOSTS}; do
     export PATH=${BASEPREFIX}/${i}/native/bin:${ORIGPATH}
-    if [ "${i}" = "riscv64-linux-gnu" ] || [ "${i}" = "powerpc64-linux-gnu" ] || [ "${i}" = "powerpc64le-linux-gnu" ]; then
+    if [ "${i}" = "powerpc64-linux-gnu" ]; then
       # Workaround for https://bugs.launchpad.net/ubuntu/+source/gcc-8-cross-ports/+bug/1853740
       # TODO: remove this when no longer needed
       HOST_LDFLAGS="${HOST_LDFLAGS_BASE} -Wl,-z,noexecstack"