GCC trunk now has multiple internal headers directories, one
containing the self-contained GCC-provided headers and one containing
the <limits.h> (not self-contained but including libc's <limits.h> or
a fixed version thereof) and the fixed headers; more such directories
may be added in future.
When glibc uses -nostdinc, it needs to use -isystem options for all
these internal directories. This patch teaches it about the
include-fixed directory (and is harmless with old GCC versions without