math.h cannot be cross-compiled

[Dima Kogan <dkogan@debian.org>]

Hi. This should probably be a bug report, but I'm not sure what package
is responsible, so I'd like to ask here first.

I just installed gcc-11-arm-linux-gnueabihf from unstable, but by
mistake I have NOT installed libc6-dev:armhf. Only the native
libc6-dev:amd64 is installed. This happens when building the one-line
"#include <math.h>" source:

  $ arm-linux-gnueabihf-gcc-11 -x c -c -o /dev/null <(echo '#include <math.h>')

  In file included from /proc/self/fd/18:1:
  /usr/include/bits/mathcalls-helper-functions.h:20:40: error: '_Float128' is not supported on this target
     20 | __MATHDECL_ALIAS (int, __fpclassify,, (_Mdouble_ __value), fpclassify)
        |                                        ^~~~~~~~~
  /usr/include/bits/mathcalls-helper-functions.h:24:37: error: '_Float128' is not supported on this target
     24 | __MATHDECL_ALIAS (int, __signbit,, (_Mdouble_ __value), signbit)

  ... many more of these ...

I guess it's picking up the amd64 headers instead of the armhf headers.
Installing libc6-dev:armhf fixes it. I would expect an error message
that says "You dummy. You need to install the arm-linux-gnueabihf flavor
of math.h". Is this a bug for libc6-dev? Also, what do my Build-Depends
need to have in order to pull in the correct header?

Thanks