linux32: add a hack to avoid redefining the type of the savefpu tag

when compiling in amd64 kernel environment with -m32.  This is a temporal
workaround for some future proper (but unclear) fix.

Reviewed by:	jhb, markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D31954

sys/modules/linux/Makefile

@@ -60,7 +60,7 @@ linux${SFX}_assym.h: linux${SFX}_genassym.o
 	sh ${SYSDIR}/kern/genassym.sh linux${SFX}_genassym.o > ${.TARGET}
 
 .if ${MACHINE_CPUARCH} == "amd64"
-VDSOFLAGS=-DCOMPAT_FREEBSD32 -DCOMPAT_LINUX32 -m32
+VDSOFLAGS=-DCOMPAT_FREEBSD32 -DCOMPAT_LINUX32 -D__M32_BROKEN_MODULE_HACK__ -m32
 .else
 VDSOFLAGS=-mregparm=0
 .endif

sys/x86/include/fpu.h

@@ -123,10 +123,12 @@ struct savexmm {
 } __aligned(16);
 
 #ifdef __i386__
+#ifndef __M32_BROKEN_MODULE_HACK__
 union savefpu {
 	struct save87	sv_87;
 	struct savexmm	sv_xmm;
 };
+#endif /* __M32_BROKEN_MODULE_HACK__ */
 #else
 /* Floating point context. (amd64 fxsave/fxrstor) */
 struct savefpu {