To recreate the problem:

$ wget https://buildroot.org/downloads/buildroot-2016.02.tar.bz2
$ tar -xjf buildroot-2016.02.tar.bz2
$ cd buildroot-2016.02
$ make raspberrypi_defconfig
$ make menuconfig
In the "Toolchain" section select "Toolchain type" "External toolchain"
and then in "Toolchain" "Musl 1.1.6 toolchain (experimental)"
Leave the configuration menu, saving the configuration
$ make

After some time you'll get:

 CC      drivers/misc/vc04_services/interface/vchiq_arm/vchiq_shim.o
 CC      fs/proc/array.o
drivers/misc/vc04_services/interface/vchiq_arm/vchiq_shim.c:42:20: fatal error: stddef.h: No such file or directory
 #include <stddef.h>
                    ^
compilation terminated.
scripts/Makefile.build:258: recipe for target 'drivers/misc/vc04_services/interface/vchiq_arm/vchiq_shim.o' failed


If I'm forced to build the system quickly (as my students are, due to short lab sessions), the only way is to switch off the EABIhf (by selection of the EABI in the "Target options", Target ABI) and use the "Toolchain (Sourcery CodeBench ARM 2014.05)"

Of course the optimal solution is to use the "Buildroot toolchain" with standard EABIhf, but it takes very long to rebuild after "make clean"...

Regards,
Wojtek