systrap: don't change an fpu state from the stub code

syshandler doesn't expect that the current fpu state can be changed.

Reported-by: syzbot+d51c9b676e1eebdfde0a@syzkaller.appspotmail.com
PiperOrigin-RevId: 547964140

pkg/sentry/platform/systrap/sysmsg/build.bzl

@@ -23,6 +23,7 @@ def cc_pie_obj(name, srcs, outs):
               ) +
               " -fno-builtin " +
               "-ffreestanding " +
+              "-mgeneral-regs-only " +
               "-g " +
               "-Wa,--noexecstack " +
               "-fno-asynchronous-unwind-tables " +