build: add linker optimization flags to gitian descriptors

Any -O argument will enable optimizations in GNU ld. We can use -O2
here, as this matches our compile flags. Note that this would also
enable additional optimizations if using the lld or gold linkers,
when compared to -O0.

contrib/gitian-descriptors/gitian-linux.yml

@@ -46,7 +46,7 @@ script: |
   FAKETIME_PROGS="date ar ranlib nm"
   HOST_CFLAGS="-O2 -g"
   HOST_CXXFLAGS="-O2 -g"
-  HOST_LDFLAGS_BASE="-static-libstdc++"
+  HOST_LDFLAGS_BASE="-static-libstdc++ -Wl,-O2"
 
   export QT_RCC_TEST=1
   export QT_RCC_SOURCE_DATE_OVERRIDE=1