Makefile: replace -O6 with -O3

-O6 is not a legal option of gcc. Unofficially gcc considers it to be
equivalent of -O3. clang chalks on it, though. This commit sets the 
default optimization flag to be -O3, like gcc actually considered it.

Review: https://reviewboard.asterisk.org/r/3280/
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Merged revisions 409308 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 409344 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 409346 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@409350 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile.rules

@@ -35,7 +35,7 @@ else
     CMD_PREFIX=
 endif
 
-OPTIMIZE?=-O6
+OPTIMIZE?=-O3
 ifneq ($(findstring darwin,$(OSARCH)),)
   ifeq ($(shell if test `/usr/bin/sw_vers -productVersion | cut -c4` -gt 5; then echo 6; else echo 0; fi),6)
     # Snow Leopard/Lion has an issue with this optimization flag on large files (like chan_sip)