Add switch to suppress hard-coded default mappings
--no_default_mappings will cause IWYU not to add the default GCC-oriented mappings, to make it easier to build custom mapping suites for other toolchains.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@457 812d1f90-f944-d88a-411a-122bdc93e1ea