Don't make clang use gcc's include-fixed

This was breaking `#include_next <limits.h>`, such that we weren't
getting definitions of `PATH_MAX` and `_POSIX_ARG_MAX`.

src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -18,16 +18,7 @@ cd clang-build
 # For whatever reason the default set of include paths for clang is different
 # than that of gcc. As a result we need to manually include our sysroot's
 # include path, /rustroot/include, to clang's default include path.
-#
-# Alsow there's this weird oddity with gcc where there's an 'include-fixed'
-# directory that it generates. It turns out [1] that Centos 5's headers are so
-# old that they're incompatible with modern C semantics. While gcc automatically
-# fixes that clang doesn't account for this. Tell clang to manually include the
-# fixed headers so we can successfully compile code later on.
-#
-# [1]: https://sourceware.org/ml/crossgcc/2008-11/msg00028.html
 INC="/rustroot/include"
-INC="$INC:/rustroot/lib/gcc/x86_64-unknown-linux-gnu/5.5.0/include-fixed"
 INC="$INC:/usr/include"
 
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