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