llvm: Link private LLVM libraries against compiler_rt for aarch64

This is required for GCC which uses libcalls for outlined atomics.

Reviewed by:	dim
Differential Revision:	https://reviews.freebsd.org/D55157

lib/clang/libclang/Makefile

@@ -14,6 +14,10 @@ PRIVATELIB=
 # Work around "relocation R_PPC_GOT16 out of range" errors
 PICFLAG=        -fPIC
 .endif
+.if ${MACHINE_CPUARCH} == "aarch64"
+# Only required for GCC
+LIBADD+=	compiler_rt
+.endif
 .endif
 
 SHARED_CXXFLAGS+=	-UPIC # To avoid compile errors

lib/clang/liblldb/Makefile

@@ -10,6 +10,10 @@ PACKAGE=	lldb
 SHLIB_CXX=	lldb
 SHLIB_MAJOR=	19
 PRIVATELIB=
+.if ${MACHINE_CPUARCH} == "aarch64"
+# Only required for GCC
+LIBADD+=	compiler_rt
+.endif
 .endif
 
 SHARED_CXXFLAGS+=	-UPIC # To avoid compile errors

lib/clang/libllvm/Makefile

@@ -14,6 +14,10 @@ PRIVATELIB=
 # Work around "relocation R_PPC_GOT16 out of range" errors
 PICFLAG=        -fPIC
 .endif
+.if ${MACHINE_CPUARCH} == "aarch64"
+# Only required for GCC
+LIBADD+=	compiler_rt
+.endif
 CFLAGS+=	-DLLVM_BUILD_LLVM_DYLIB
 .endif