Add InstrProfilingPlatformFuchsia.c to profiler_builtins
Add InstrProfilingPlatformFuchsia.c to profiler_builtins

All other Platform files included in `llvm-project/compiler-rt` were
present, except Fuchsia.

Now that there is a functional end-to-end version of
`-Zinstrument-coverage`, I need to start building and testing
coverage-enabled Rust programs on Fuchsia, and this file is required.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust