math/gsl: fix build on powerpc64 elfv2

Clang 10 fails with assertion error (already reported):
/bin/sh ../libtool  --tag=CC    --mode=compile cc -DHAVE_CONFIG_H  -I. -I..  -I..    -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing -MT cgbmv.lo -MD -MP -MF .deps/cgbmv.Tpo -c -o cgbmv.lo cgbmv.c
libtool: compile:  cc -DHAVE_CONFIG_H -I. -I.. -I.. -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -MT cgbmv.lo -MD -MP -MF .deps/cgbmv.Tpo -c cgbmv.c  -fPIC -DPIC -o .libs/cgbmv.o
Assertion failed: (HT.TopLevelMap[ThisEntry->getKey()] == ThisEntry && "Scope imbalance!"), function ~ScopedHashTableScope, file /usr/src/contrib/llvm-project/llvm/include/llvm/ADT/ScopedHashTable.h, line 245.
Stack dump:
0.      Program arguments: cc -DHAVE_CONFIG_H -I. -I.. -I.. -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -MT cgbmv.lo -MD -MP -MF .deps/cgbmv.Tpo -c cgbmv.c -fPIC -DPIC -o .libs/cgbmv.o
1.      <eof> parser at end of file
2.      Code generation
3.      Running pass 'Function Pass Manager' on module 'cgbmv.c'.
4.      Running pass 'Early CSE' on function '@cblas_cgbmv'
#0 0x0000000013bac208 PrintStackTrace /usr/src/contrib/llvm-project/llvm/lib/Support/Unix/Signals.inc:564:13
#1 0x0000000013ba98d0 RunSignalHandlers /usr/src/contrib/llvm-project/llvm/lib/Support/Signals.cpp:67:5
#2 0x0000000013baf278 HandleCrash /usr/src/contrib/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:75:7
#3 0x0000000013baf4ec CrashRecoverySignalHandler /usr/src/contrib/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:0:51
#4 0x0000000815732748 handle_signal /usr/src/lib/libthr/thread/thr_sig.c:303:3
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

math/gsl/Makefile

@@ -24,8 +24,14 @@ INFO=		gsl-ref
 CORELIMIT?=	/usr/bin/limits -Sc 0
 .endif
 
+.include <bsd.port.pre.mk>
+
+.if ${ARCH} == powerpc64
+USE_GCC=	any
+.endif
+
 check regression-test test: build
 	@cd ${WRKSRC}; ${SETENV} ${MAKE_ENV} ${CORELIMIT} ${MAKE_CMD} \
 	${MAKE_ARGS} check
 
-.include <bsd.port.mk>
+.include <bsd.port.post.mk>