Add initial support -fcoverage-mapping support

This is enought make it work up until llvm-cov tries to read the
named data sections in the binary and can't find them.  For this
final part to work we probably need to switch the object format to
using multiple code and data sections:
WebAssembly/tool-conventions#138

Not sure if its worth submitting this part in isolation without
a fully working solution?

See #13046

Author: sbc100

emcc.py

@@ -1374,7 +1374,6 @@ def filter_out_duplicate_dynamic_libs(inputs):
     if not compile_only and not options.post_link:
       ldflags = shared.emsdk_ldflags(newargs)
       for f in ldflags:
-        newargs.append(f)
         add_link_flag(len(newargs), f)
 
     # SSEx is implemented on top of SIMD128 instruction set, but do not pass SSE flags to LLVM
@@ -1909,9 +1908,6 @@ def get_full_import_name(name):
                                              '_emscripten_stack_get_base',
                                              '_emscripten_stack_get_end']
 
-    if shared.Settings.USE_PTHREADS:
-      newargs.append('-pthread')
-
     # Any "pointers" passed to JS will now be i64's, in both modes.
     if shared.Settings.MEMORY64:
       if settings_key_changes.get('WASM_BIGINT') == '0':