STABLE fails since change to Clang 8

[Eugen]

I used to build world and kernel every other week.
My last successful build of the source tree was version 345754.
I believe in the meantime Clang was changed from 7 to 8.
World build fails ever since with this:
---------------------------------------------------------------------------------------------------
--- Basic/Targets/AArch64.o ---
c++  -O2 -pipe -I/home/sys_build/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libclang
-I/home/sys_build/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libllvm
-I/usr/src/contrib/llvm/tools/clang/lib/Basic
-I/usr/src/contrib/llvm/tools/clang/lib/Driver
-I/usr/src/contrib/llvm/tools/clang/include
-I/usr/src/lib/clang/include -I/usr/src/contrib/llvm/include
-DLLVM_BUILD_GLOBAL_ISEL -D__STDC_LIMIT_MACROS
-D__STDC_CONSTANT_MACROS -DNDEBUG
-DLLVM_DEFAULT_TARGET_TRIPLE=\"x86_64-unknown-freebsd12.0\"
-DLLVM_HOST_TRIPLE=\"x86_64-unknown-freebsd12.0\"
-DDEFAULT_SYSROOT=\"/home/sys_build/usr/src/amd64.amd64/tmp\"
-DLLVM_TARGET_ENABLE_X86
-DLLVM_NATIVE_ASMPARSER=LLVMInitializeX86AsmParser
-DLLVM_NATIVE_ASMPRINTER=LLVMInitializeX86AsmPrinter
-DLLVM_NATIVE_DISASSEMBLER=LLVMInitializeX86Disassembler
-DLLVM_NATIVE_TARGET=LLVMInitializeX86Target
-DLLVM_NATIVE_TARGETINFO=LLVMInitializeX86TargetInfo
-DLLVM_NATIVE_TARGETMC=LLVMInitializeX86TargetMC -ffunction-sections
-fdata-sections -gline-tables-only -MD
-MF.depend.Basic_Targets_AArch64.o -MTBasic/Targets/AArch64.o
-Qunused-arguments
-I/home/sys_build/usr/src/amd64.amd64/tmp/legacy/usr/include
-std=c++11 -fno-exceptions -fno-rtti -gline-tables-only -stdlib=libc++
-Wno-c++11-extensions  -c
/usr/src/contrib/llvm/tools/clang/lib/Basic/Targets/AArch64.cpp -o
Basic/Targets/AArch64.o
--- Basic/Targets/AMDGPU.o ---
c++  -O2 -pipe -I/home/sys_build/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libclang
-I/home/sys_build/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libllvm
-I/usr/src/contrib/llvm/tools/clang/lib/Basic
-I/usr/src/contrib/llvm/tools/clang/lib/Driver
-I/usr/src/contrib/llvm/tools/clang/include
-I/usr/src/lib/clang/include -I/usr/src/contrib/llvm/include
-DLLVM_BUILD_GLOBAL_ISEL -D__STDC_LIMIT_MACROS
-D__STDC_CONSTANT_MACROS -DNDEBUG
-DLLVM_DEFAULT_TARGET_TRIPLE=\"x86_64-unknown-freebsd12.0\"
-DLLVM_HOST_TRIPLE=\"x86_64-unknown-freebsd12.0\"
-DDEFAULT_SYSROOT=\"/home/sys_build/usr/src/amd64.amd64/tmp\"
-DLLVM_TARGET_ENABLE_X86
-DLLVM_NATIVE_ASMPARSER=LLVMInitializeX86AsmParser
-DLLVM_NATIVE_ASMPRINTER=LLVMInitializeX86AsmPrinter
-DLLVM_NATIVE_DISASSEMBLER=LLVMInitializeX86Disassembler
-DLLVM_NATIVE_TARGET=LLVMInitializeX86Target
-DLLVM_NATIVE_TARGETINFO=LLVMInitializeX86TargetInfo
-DLLVM_NATIVE_TARGETMC=LLVMInitializeX86TargetMC -ffunction-sections
-fdata-sections -gline-tables-only -MD
-MF.depend.Basic_Targets_AMDGPU.o -MTBasic/Targets/AMDGPU.o
-Qunused-arguments
-I/home/sys_build/usr/src/amd64.amd64/tmp/legacy/usr/include
-std=c++11 -fno-exceptions -fno-rtti -gline-tables-only -stdlib=libc++
-Wno-c++11-extensions  -c
/usr/src/contrib/llvm/tools/clang/lib/Basic/Targets/AMDGPU.cpp -o
Basic/Targets/AMDGPU.o
--- Basic/SourceManager.o ---
/usr/src/contrib/llvm/tools/clang/lib/Basic/SourceManager.cpp:1195:10:
fatal error: 'emmintrin.h' file not found
#include <emmintrin.h>
         ^~~~~~~~~~~~~
1 error generated.
*** [Basic/SourceManager.o] Error code 1

make[4]: stopped in /usr/src/lib/clang/libclang
1 error

make[4]: stopped in /usr/src/lib/clang/libclang
*** [all_subdir_lib/clang/libclang] Error code 2

make[3]: stopped in /usr/src/lib/clang
1 error

make[3]: stopped in /usr/src/lib/clang
*** [cross-tools] Error code 2

make[2]: stopped in /usr/src
1 error

make[2]: stopped in /usr/src
*** [_cross-tools] Error code 2

make[1]: stopped in /usr/src
1 error
---------------------------------------------------------------------------------------------------
uname -a produces this:

FreeBSD main 12.0-STABLE FreeBSD 12.0-STABLE #0 r345754M: Sun Mar 31
10:01:26 PDT 2019
root at main:/home/sys_build/usr/src/amd64.amd64/sys/WORLD  amd64

llvm 7 and 8 also fail in ports.

I don't know why it even tries to build an AMDGPU target, as my system
doesn't have one! My GPU is Nvidia.

Thanks for any hints that may fix this.
Eugen