It is no longer possible to remove the SSE2 requirement.

So we remove the JIT entirely on pmmx/x86.

--- qtdeclarative-everywhere-src-5.12.6/src/qml/jsruntime/qv4global_p.h.old	2019-11-07 11:01:48.000000000 +0000
+++ qtdeclarative-everywhere-src-5.12.6/src/qml/jsruntime/qv4global_p.h	2019-12-27 06:11:55.456270846 +0000
@@ -88,10 +88,7 @@
 //
 // NOTE: This should match the logic in qv4targetplatform_p.h!
 
-#if defined(Q_PROCESSOR_X86_32) && (QT_POINTER_SIZE == 4) \
-    && (defined(Q_OS_WIN) || defined(Q_OS_LINUX) || defined(Q_OS_QNX) || defined(Q_OS_FREEBSD))
-#  define V4_ENABLE_JIT
-#elif defined(Q_PROCESSOR_X86_64) && (QT_POINTER_SIZE == 8) \
+#if defined(Q_PROCESSOR_X86_64) && (QT_POINTER_SIZE == 8) \
     && (defined(Q_OS_WIN) || defined(Q_OS_LINUX) || defined(Q_OS_QNX) || defined(Q_OS_MAC) || defined(Q_OS_FREEBSD))
 #  define V4_ENABLE_JIT
 #elif defined(Q_PROCESSOR_ARM_32) && (QT_POINTER_SIZE == 4) \
--- qtdeclarative-everywhere-src-5.12.6/src/qml/qml/v8/qv8engine.cpp.old	2019-11-07 11:01:48.000000000 +0000
+++ qtdeclarative-everywhere-src-5.12.6/src/qml/qml/v8/qv8engine.cpp	2020-08-09 17:28:54.028331206 +0000
@@ -138,14 +138,6 @@
     , m_xmlHttpRequestData(nullptr)
 #endif
 {
-#ifndef Q_OS_WASM // wasm does not have working simd QTBUG-63924
-#ifdef Q_PROCESSOR_X86_32
-    if (!qCpuHasFeature(SSE2)) {
-        qFatal("This program requires an X86 processor that supports SSE2 extension, at least a Pentium 4 or newer");
-    }
-#endif
-#endif
-
     QML_MEMORY_SCOPE_STRING("QV8Engine::QV8Engine");
     qMetaTypeId<QJSValue>();
     qMetaTypeId<QList<int> >();