Tested on fedora 29 and fedora 30 (rawhide), initial attempts to build qtwebengine-5.12.0 on i686 arch fails with error:

In file included from ../../3rdparty/chromium/mojo/public/c/system/types.h:15,
from ../../3rdparty/chromium/mojo/core/handle_signals_state.h:9,
from ../../3rdparty/chromium/mojo/core/dispatcher.h:19,
from ../../3rdparty/chromium/mojo/core/platform_handle_dispatcher.h:11,
from ../../3rdparty/chromium/mojo/core/platform_handle_dispatcher.cc:5:
../../3rdparty/chromium/mojo/public/c/system/buffer.h:33:42: error: static assertion failed: int64_t has weird alignment
MOJO_STATIC_ASSERT(MOJO_ALIGNOF(int64_t) == 8, "int64_t has weird alignment");
../../3rdparty/chromium/mojo/public/c/system/macros.h:15:53: note: in definition of macro 'MOJO_STATIC_ASSERT'
#define MOJO_STATIC_ASSERT(expr, msg) static_assert(expr, msg)
^~~~
../../3rdparty/chromium/mojo/public/c/system/data_pipe.h:43:42: error: static assertion failed: int64_t has weird alignment
MOJO_STATIC_ASSERT(MOJO_ALIGNOF(int64_t) == 8, "int64_t has weird alignment");
../../3rdparty/chromium/mojo/public/c/system/macros.h:15:53: note: in definition of macro 'MOJO_STATIC_ASSERT'
#define MOJO_STATIC_ASSERT(expr, msg) static_assert(expr, msg)
^~~~
../../3rdparty/chromium/mojo/public/c/system/message_pipe.h:38:42: error: static assertion failed: int64_t has weird alignment
MOJO_STATIC_ASSERT(MOJO_ALIGNOF(int64_t) == 8, "int64_t has weird alignment");
../../3rdparty/chromium/mojo/public/c/system/macros.h:15:53: note: in definition of macro 'MOJO_STATIC_ASSERT'
#define MOJO_STATIC_ASSERT(expr, msg) static_assert(expr, msg)