Add -mlong-calls also to LLVM's STATIC_CFLAGS on ARM

-mlong-calls was set only in STATIC_CXXFLAGS, but there are some .c
source files in LLVM which also need -mlong-calls.

Unfortunately this is not sufficient to fix linking lldb on ARM,
because LLVM-generated calls to __aeabi_read_tp do not honour the
-mlong-calls flag.  See LLVM PR31769 for details.

Reviewed by:	dim
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D9348

lib/clang/llvm.build.mk

@@ -46,5 +46,6 @@ CXXFLAGS+=	-fno-rtti
 CXXFLAGS.clang+= -stdlib=libc++
 
 .if ${MACHINE_CPUARCH} == "arm"
+STATIC_CFLAGS+= -mlong-calls
 STATIC_CXXFLAGS+= -mlong-calls
 .endif