RPI3 can't build kernel-toolchain

[bob prohaska]

On Mon, Feb 12, 2018 at 10:32:56AM -0800, bob prohaska wrote:
> On a Pi3 running r328935 trying to build sources at 329171 a 
> make -j2 kernel-toolchain fails with
> /usr/bin/ld: error: duplicate symbol: 
> >>> defined at ASTImporter.o:() in archive /usr/obj/usr/src/arm64.aarch64/tmp/obj-tools/lib/clang/libclang/libclang.a
> 

Updating sources eventually allowed make kernel-toolchain to build without
errors. However, make buildkernel still stops, reporting

In file included from /usr/src/sys/crypto/armv8/armv8_crypto_wrap.c:46:
/usr/obj/usr/src/arm64.aarch64/tmp/usr/lib/clang/6.0.0/include/arm_neon.h:31:10: fatal error: 'stdint.h' file not found
#include <stdint.h>
         ^~~~~~~~~~
1 error generated.
*** [armv8_crypto_wrap.o] Error code 1

This seems rather odd, since find reports files with that name in
several locations within /usr/src:

root at www:/usr/src # find . -name stdint.h -depth -print
./sys/sys/stdint.h
./sys/contrib/zstd/lib/freebsd/stdint.h
./contrib/llvm/tools/clang/lib/Headers/stdint.h
./contrib/libc++/include/stdint.h
./contrib/libstdc++/include/tr1/stdint.h

To the best of my ability the source tree is unmolested, svnlite info reports:
root at www:/usr/src # svnlite status .
?       buildkernel.log
?       buildscript
?       installscript
?       kernelscript
?       toolchain.log
?       toolchainscript
root at www:/usr/src # 

The source tree is presently at r329360.

Thanks for reading, and any help!

bob prohaska