Small change to CMake options to only enable the PreferredToolArchitecture=x64 option (required for 32bit TeamCity builds) on 32 bit builds generally - removes a ton of 'unknown option, ignored' spam in the build output

to avoid TeamCity build failures due to 32-bit compiler running out of virtual
memory for precompiled headers.

for Windows

--HG--
branch : Viewer-Build

The viewer's 00-COMPILE-LINK-RUN.txt recommends passing -gdwarf-2 to the Mac
compiler, and so we've been doing ever since before the viewer-build-variables
repo was engaged. Now we discover that when CMake sees -gdwarf-2, it removes
the -g switch entirely. It also removes it when you pass plain -g. Only when
you pass -gdwarf-with-dsym or just -gdwarf does CMake pass plain -g to the
compiler. Change -gdwarf-2, if specified, to -gdwarf so we at least get -g.

Going forward, the intention is to set in 00-Common.cmake only switches not
already set for ALL viewer-related libraries in
https://bitbucket.org/lindenlab/viewer-build-variables/src/tip/variables.

To that end, remove all switches redundant with settings from that file.
Remove redundancies within 00-Common.cmake.
Remove cruft testing for gcc versions older than 4.3.