Add support for macOS

To support dynamic code coverage on macOS we need nuget with CLR IE macOS binaries.

I was trying to build CLR IE on macOS but it looks like it's not so easy. I was trying to build it with clang 11. I saw some issues with stringstream but after commenting out some stringstream usages I was able to reach linking. Linking is failing with such issues:

ld: warning: ignoring file ../unix/src/liblinux_pal.a, building for macOS-x86_64 but attempting to link with file built for macOS-x86_64
ld: warning: ignoring file ../unix/src/atl/libatl.a, building for macOS-x86_64 but attempting to link with file built for macOS-x86_64
ld: warning: ignoring file ../InstrumentationEngine.Api/libInstrumentationEngine.Api.a, building for macOS-x86_64 but attempting to link with file built for unknown-unsupported file format ( 0x21 0x3C 0x61 0x72 0x63 0x68 0x3E 0x0A 0x2F 0x20 0x20 0x20 0x20 0x20 0x20 0x20 )
ld: warning: ignoring file ../InstrumentationEngine.Lib/libInstrumentationEngine.Lib.a, building for macOS-x86_64 but attempting to link with file built for unknown-unsupported file format ( 0x21 0x3C 0x61 0x72 0x63 0x68 0x3E 0x0A 0x2F 0x20 0x20 0x20 0x20 0x20 0x20 0x20 )

There are also issues with objcopy in macOS. I've tried 2 versions llvm-objcopy from llvm 11, and gobjcopy from binutils package. In both cases during linking there were issues and redefined library was ignored.

All my changes you can find on branch: https://github.com/jakubch1/CLRInstrumentationEngine/tree/dev/jachocho/macos

Start by reviewing the dev/jachocho/macos branch linked in the issue and reproduce the clang/linker and objcopy failures described there. Trace how the existing CLR Instrumentation Engine libraries are built and packaged, then determine the macOS-compatible build path. Done means dynamic code coverage works on macOS and the required macOS binaries are included in NuGet.