A 64-bit Linux distro I run in VirtualBox updated from clang 3.9 to 6.0, so I recompiled some code to compare the differences. The code compiled with --cc:clang on 3.9, but using 6.0 it gives the following error output.
Hint: [Link]
/usr/bin/ld: cannot find crtbegin.o: No such file or directory
clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation)
Error: execution of an external program failed: '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'
[jzakiya@localhost nim]$
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