WIP Reduce advzip compression to -z3 (bug workaround)

As of GDB 13.1, gdb.exe contains static data that triggers a bug in x64
Windows zip integration as far back as Windows 7. After recompressing
with "advzip -z4", extracting gdb.exe fails with "Unspecified error"
0x80004005. It happens with both 32-bit and 64-bit gdb.exe builds, and
regardless of compiler flags, which is why I'm confident it's static
data. No other zip program I've tried has trouble with this file.

WIP NOTE: I thought it might not happen with -z3, but it still does. The
committed "advzip.sample" contains the bad pattern.

Author: skeeto

advzip.sample

@@ -101,7 +101,7 @@ done
 
 if [ $compact = yes ]; then
     printf "%s$suffix.zip\n" $builds \
-        | xargs -I{} -P$(nproc) $dryrun advzip -z4 {}
+        | xargs -I{} -P$(nproc) $dryrun advzip -z3 {}
 fi
 
 cleanup