Toolchains: Add C++98 as a language standard.

This was curiously missing from the CompilerFlag enum, so e.g. setting
"QMAKE_CXXFLAGS += -std=c++98" in a .pro file did not have the desired
effect on higher-level code such as the code model.

Change-Id: Ia4323b699a2ccea9d3cbde10b05affe87d955277
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>