It looks like your clang 6.0 installation is broken.
If you check this line you will not see any mentions on `crtbegin.o`.
clang -o /home/jzakiya/nim/twinprimes_test4a
/home/jzakiya/nim/nimcache/twinprimes_test4a.o
/home/jzakiya/nim/nimcache/stdlib_system.o
/home/jzakiya/nim/nimcache/stdlib_sharedlist.o
/home/jzakiya/nim/nimcache/stdlib_locks.o
/home/jzakiya/nim/nimcache/stdlib_math.o
/home/jzakiya/nim/nimcache/stdlib_strutils.o
/home/jzakiya/nim/nimcache/stdlib_parseutils.o
/home/jzakiya/nim/nimcache/stdlib_algorithm.o
/home/jzakiya/nim/nimcache/stdlib_typetraits.o
/home/jzakiya/nim/nimcache/stdlib_times.o
/home/jzakiya/nim/nimcache/stdlib_posix.o
/home/jzakiya/nim/nimcache/stdlib_os.o
/home/jzakiya/nim/nimcache/stdlib_ospaths.o
/home/jzakiya/nim/nimcache/stdlib_osproc.o
/home/jzakiya/nim/nimcache/stdlib_strtabs.o
/home/jzakiya/nim/nimcache/stdlib_hashes.o
/home/jzakiya/nim/nimcache/stdlib_streams.o
/home/jzakiya/nim/nimcache/stdlib_cpuinfo.o
/home/jzakiya/nim/nimcache/stdlib_linux.o
/home/jzakiya/nim/nimcache/stdlib_threadpool.o
/home/jzakiya/nim/nimcache/stdlib_cpuload.o
/home/jzakiya/nim/nimcache/stdlib_tables.o
-pthread -lm -lrt -ldl
