Makefile: replace -O6 with -O3

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@409346 65c4cc65-6c06-0410-ace0-fbb531ad65f3