Update test scripts and Makefiles to allow testing with GCC:
- introduce new variable ARCHFLAG in make/Makefile.rules to switch between "-arch" on Mac and "-m" everywhere else
- update testcase makefiles to use LD_EXTRAS instead of LDFLAGS (the former interacts with Makefile.rules badly)
- special treatment for gcc 4.6: replace "-std=c++11" with "-std=c++0x" as the former is not handled correctly
- remove hardcoded "-arch" from test Makefile

This patch should not have any effect on lldb on Mac OS X.