x11/babl: fix build on powerpc64 elfv2

Clang 10 hits assertion error:
Assertion failed: (!IsStrict && "Don't know how to expand for strict nodes."), function ExpandNode, file /poudriere/jails/powerpc64-head/usr/src/contrib/llvm-project/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp, line 3638.
Stack dump:
0.	Program arguments: cc -Ibabl/4170c83@@babl-0.1@sha -Ibabl -I../babl -I. -I../ -Ibabl/base -I../babl/base -I/usr/local/include -Xclang -fcolor-diagnostics -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O3 -Ofast -fno-unsafe-math-optimizations -Wdeclaration-after-statement -Winit-self -Wmissing-declarations -Wmissing-prototypes -Wold-style-definition -Wpointer-arith -O2 -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -isystem /usr/local/include -fPIC -pthread -DLIBDIR="/usr/local/lib" -MD -MQ babl/4170c83@@babl-0.1@sha/babl-model.c.o -MF babl/4170c83@@babl-0.1@sha/babl-model.c.o.d -o babl/4170c83@@babl-0.1@sha/babl-model.c.o -c ../babl/babl-model.c
1.	<eof> parser at end of file
2.	Code generation
3.	Running pass 'Function Pass Manager' on module '../babl/babl-model.c'.
4.	Running pass 'PowerPC DAG->DAG Pattern Instruction Selection' on function '@babl_model_is_symmetric'
#0 0x0000000013bba6c8 (/usr/bin/cc+0x13bba6c8)
#1 0x0000000013bb7d90 (/usr/bin/cc+0x13bb7d90)
#2 0x0000000013bbd738 (/usr/bin/cc+0x13bbd738)
#3 0x0000000013bbd9ac (/usr/bin/cc+0x13bbd9ac)
#4 0x0000000815740838 (/lib/libthr.so.3+0x27838)
cc: error: clang frontend command failed due to signal (use -v to see invocation)
FreeBSD clang version 10.0.0 (git@github.com:llvm/llvm-project.git llvmorg-10.0.0-rc3-1-gc290cb61fdc)
Target: powerpc64-unknown-freebsd13.0
Thread model: posix
InstalledDir: /usr/bin

Use GCC for now.

x11/babl/Makefile

@@ -27,4 +27,10 @@ SIMD_MESON_OFF=	-Denable-mmx=false -Denable-sse=false \
 		-Denable-sse4_1=false -Denable-avx2=false \
 		-Denable-f16c=false
 
-.include <bsd.port.mk>
+.include <bsd.port.pre.mk>
+
+.if ${ARCH} == powerpc64
+USE_GCC=	yes
+.endif
+
+.include <bsd.port.post.mk>