This is on a 32-bit AMD Sempron 3000+ PC, and I suspect it may have something to do with lacking SSE2 support since that is an occasional issue with newer software, though I thought those issues had been dealt with upstream. In any case, below is the output. I also had the same failure with qtgui-5.9.2. There was also a Gentoo discussion about apparently the same issue with qtgui-4 a while back: https://forums.gentoo.org/viewtopic-t-1055316-start-0.html

i686-pc-linux-gnu-ar cqs ../../../lib/libQt5AccessibilitySupport.a .obj/qaccessiblebridgeutils.o
make[1]: Leaving directory '/var/tmp/portage/dev-qt/qtgui-5.9.4-r3/work/qtbase-opensource-src-5.9.4/src/platformsupport/accessibility'
cd linuxaccessibility/ && ( test -e Makefile || /var/tmp/portage/dev-qt/qtgui-5.9.4-r3/work/qtbase-opensource-src-5.9.4/bin/qmake -o Makefile /var/tmp/portag
e/dev-qt/qtgui-5.9.4-r3/work/qtbase-opensource-src-5.9.4/src/platformsupport/linuxaccessibility/linuxaccessibility.pro CONFIG+=release CONFIG-=debug 'QMAKE_A
R=i686-pc-linux-gnu-ar cqs' QMAKE_CC=i686-pc-linux-gnu-gcc QMAKE_LINK_C=i686-pc-linux-gnu-gcc QMAKE_LINK_C_SHLIB=i686-pc-linux-gnu-gcc QMAKE_CXX=i686-pc-linu
x-gnu-g++ QMAKE_LINK=i686-pc-linux-gnu-g++ QMAKE_LINK_SHLIB=i686-pc-linux-gnu-g++ QMAKE_OBJCOPY=i686-pc-linux-gnu-objcopy QMAKE_RANLIB= QMAKE_STRIP=i686-pc-l
inux-gnu-strip 'QMAKE_CFLAGS=-march=athlon-xp -O2 -mno-sse2 -pipe' QMAKE_CFLAGS_RELEASE= QMAKE_CFLAGS_DEBUG= 'QMAKE_CXXFLAGS=-march=athlon-xp -O2 -mno-sse2 -
pipe' QMAKE_CXXFLAGS_RELEASE= QMAKE_CXXFLAGS_DEBUG= 'QMAKE_LFLAGS=Wl,-O1 -Wl,sort-common -Wl,-as-needed' QMAKE_LFLAGS_RELEASE= QMAKE_LFLAGS_DEBUG= ) && m
ake -f Makefile  
make[1]: Entering directory '/var/tmp/portage/dev-qt/qtgui-5.9.4-r3/work/qtbase-opensource-src-5.9.4/src/platformsupport/linuxaccessibility'
/usr/lib/qt5/bin/qdbusxml2cpp -i struct_marshallers_p.h -a deviceeventcontroller_adaptor.h: dbusxml/DeviceEventController.xml
make[1]: *** [Makefile:1382: deviceeventcontroller_adaptor.h] Illegal instruction
make[1]: Leaving directory '/var/tmp/portage/dev-qt/qtgui-5.9.4-r3/work/qtbase-opensource-src-5.9.4/src/platformsupport/linuxaccessibility'
make: *** [Makefile:331: sub-linuxaccessibility-make_first] Error 2

dmesg also shows the following output:

[427466.596145] traps: qdbusxml2cpp[21254] trap invalid opcode ip:b7b09e7b sp:bffcdee0 error:0 in libQt5Core.so.5.9.4[b7a4f000+48c000]