emulators/qemu: fix build on powerpc

Define _CALL_SYSV to fix error:
In file included from /wrkdirs/usr/ports/emulators/qemu/work/qemu-5.0.1/tcg/tcg.c:344:                                                                                                                                                      
/wrkdirs/usr/ports/emulators/qemu/work/qemu-5.0.1/tcg/ppc/tcg-target.inc.c:2290:3: error: "Unhandled abi"

Use gcc because clang hangs.

emulators/qemu/Makefile

@@ -123,7 +123,8 @@ PORTDOCS?=	docs index.html interop/.buildinfo interop/* qemu-ga-ref.html qemu-ga
 
 WITHOUT_CPU_CFLAGS=yes	#to avoid problems with register allocation
 CFLAGS:=	${CFLAGS:C/-fno-tree-vrp//}
-CFLAGS_amd64+=	-fPIC
+CFLAGS_amd64=	-fPIC
+CFLAGS_powerpc=	-D_CALL_SYSV
 CONFIGURE_ARGS?=--localstatedir=/var \
 		--extra-ldflags=-L${LOCALBASE}/lib --disable-libssh \
 		--prefix=${PREFIX} --cc=${CC} --enable-netmap --disable-kvm \
@@ -143,6 +144,7 @@ MAKE_ARGS+=	ARCH=x86_64
 
 .if ${ARCH} == "powerpc"
 MAKE_ARGS+=	ARCH=ppc
+USE_GCC=	yes
 .endif
 
 .if ${ARCH} == "powerpc64"