Unable to build 12-current/amd64

Since r326496, buildworld on my 12-current/amd64 system has consistently
died as follows.  I have no problems building on i386 or building
12-current/amd64 on 11-stable.

...
>>> stage 3: cross tools
--------------------------------------------------------------
cd /usr/src; INSTALL="sh /usr/src/tools/install.sh"  TOOLS_PREFIX=/usr/obj/usr/src/amd64.amd64/tmp  PATH=/usr/obj/usr/src/amd64.amd64/tmp/legacy/usr/sbin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/usr/bin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/bin:/sbin:/bin:/usr/sbin:/usr/bin  WORLDTMP=/usr/obj/usr/src/amd64.amd64/tmp  MAKEFLAGS="-m /usr/src/tools/build/mk  -m /usr/src/share/mk" make  -f Makefile.inc1  DESTDIR=  OBJTOP='/usr/obj/usr/src/amd64.amd64/tmp/obj-tools'  OBJROOT='${OBJTOP}/'  MAKEOBJDIRPREFIX=  BOOTSTRAPPING=1200054  BWPHASE=cross-tools  SSP_CFLAGS=  MK_HTML=no NO_LINT=yes MK_MAN=no  -DNO_PIC MK_PROFILE=no -DNO_SHARED  -DNO_CPU_CFLAGS MK_WARNS=no MK_CTF=no  MK_CLANG_EXTRAS=no MK_CLANG_FULL=no  MK_LLDB=no MK_TESTS=no  MK_INCLUDES=yes  TARGET=amd64 TARGET_ARCH=amd64  MK_GDB=no MK_LLD_IS_LD=no MK_TESTS=no cross-tools
...
===> lib/clang/libclang (all)
...
c++  -O2 -pipe -I/usr/obj/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libclang -I/usr/obj/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libllvm -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 -DLLVM_DEFAULT_TARGET_TRIPLE=\"x86_64-unknown-freebsd12.0\" -DLLVM_HOST_TRIPLE=\"x86_64-unknown-freebsd12.0\" -DDEFAULT_SYSROOT=\"/usr/obj/usr/src/amd64.amd64/tmp\" -ffunction-sections -fdata-sections -gline-tables-only -MD -MF.depend.Basic_SourceLocation.o -MTBasic/SourceLocation.o -Qunused-arguments -I/usr/obj/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/SourceLocation.cpp -o Basic/SourceLocation.o
c++  -O2 -pipe -I/usr/obj/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libclang -I/usr/obj/usr/src/amd64.amd64/tmp/obj-tools/lib/clang/libllvm -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 -DLLVM_DEFAULT_TARGET_TRIPLE=\"x86_64-unknown-freebsd12.0\" -DLLVM_HOST_TRIPLE=\"x86_64-unknown-freebsd12.0\" -DDEFAULT_SYSROOT=\"/usr/obj/usr/src/amd64.amd64/tmp\" -ffunction-sections -fdata-sections -gline-tables-only -MD -MF.depend.Basic_SourceManager.o -MTBasic/SourceManager.o -Qunused-arguments -I/usr/obj/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/SourceManager.cpp -o Basic/SourceManager.o
/usr/src/contrib/llvm/tools/clang/lib/Basic/SourceManager.cpp:1166:10: fatal error: 'emmintrin.h' file not found
#include <emmintrin.h>
         ^~~~~~~~~~~~~
1 error generated.
*** Error code 1

Stop.
make[4]: stopped in /usr/src/lib/clang/libclang

I'm building on a 12.0-CURRENT VirtualBox guest at r326430.  I've checked
that my /usr/src is clean and deleted /usr/obj to no effect.  I have dug
into SourceManager.cpp and the #include is protected by a #if __SSE2__,
which is relying on clang internal checks to define (and my CPU supports
SSE2).  Does anyone have any ideas to explain what is going on?

-- 
Peter Jeremy