Premake unused -rdynamic for clang

[elexis]

With clang 8 on linux I get this error spammed for every file:

clang-8: warning: argument unused during compilation: '-rdynamic' [-Wunused-command-line-argument]

for example when compiling the TU of atlas Object.cpp

It seems the setting has a different name for clang.

While I find the setting in the specs: ​https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-rdynamic

I also find multiple references saying one should use `-Wl,-export_dynamic ` instead, for example ​https://keitaito.com/blog/2017/01/02/exploring-compiler-options.html.

See also #1179.