guix: re-revert riscv execstack workaround

Now that we use GCC 10 for release builds, we no-longer need to
pass-Wl,-z,noexecstack to get a non-executable stack in RISC-V binaries.

This was originally removed in bitcoin#21036, but then re-added in bitcoin#21799, when
we reverted to using GCC 8.

Author: fanquake

contrib/guix/libexec/build.sh

@@ -250,7 +250,7 @@ case "$HOST" in
 esac
 
 case "$HOST" in
-    powerpc64-linux-*|riscv64-linux-*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,-z,noexecstack" ;;
+    powerpc64-linux-*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,-z,noexecstack" ;;
 esac
 
 # Make $HOST-specific native binaries from depends available in $PATH