Fix cpp_sys_macro_p with -ftrack-macro-expansion

Fix cpp_sys_macro_p with -ftrack-macro-expansion

cpp_sys_macro_p crashes when -ftrack-macro-expansion is on.  The issue
can be reproduced by running the tests:

    runtest --tool gcc --tool_opts="-ftrack-macro-expansion" cpp.exp=sysmac1.c
    runtest --tool gcc --tool_opts="-ftrack-macro-expansion" cpp.exp=sysmac2.c

This is because it just doesn't support that mode.  Fixed thus.
Tested and bootstrapped on x86_64-unknown-linux-gnu against trunk.

Note that the bootstrap with -ftrack-macro-expansion turned on
exhibits other separate issues that are addressed in subsequent
patches.  This patch just fixes one class of problems.

The patch does pass bootstrap with -ftrack-macro-expansion turned off,
though.

libcpp/

	* macro.c (cpp_sys_macro_p):  Support -ftrack-macro-expansion.

From-SVN: r186965