Bug 887902 - move --enable-debug{,-symbols} option processing before MOZ_COMPILER_OPTS; r=glandium

MOZ_COMPILER_OPTS depends on MOZ_DEBUG_FLAGS being set for proper checking of
linker dead code deletion when using GCC.

GC'ing sections saves ~1.5MB off of mobile libxul according to size(1):

[froydnj@cerebro eideticker]$ size /opt/build/froydnj/build-android/dist/bin/libxul.so
   text	   data	    bss	    dec	    hex	filename
23733400	2314816	1495973	27544189	1a44a7d	/opt/build/froydnj/build-android/dist/bin/libxul.so
[froydnj@cerebro eideticker]$ size /opt/build/froydnj/build-android/dist/bin/libxul.so
   text	   data	    bss	    dec	    hex	filename
22261936	2145116	1494533	25901585	18b3a11	/opt/build/froydnj/build-android/dist/bin/libxul.so