Add profiling support, through the rustc -Z profile flag.

When -Z profile is passed, the GCDAProfiling LLVM pass is added
to the pipeline, which uses debug information to instrument the IR.
After compiling with -Z profile, the $(OUT_DIR)/$(CRATE_NAME).gcno
file is created, containing initial profiling information.
After running the program built, the $(OUT_DIR)/$(CRATE_NAME).gcda
file is created, containing branch counters.

The created *.gcno and *.gcda files can be processed using
the "llvm-cov gcov" and "lcov" tools. The profiling data LLVM
generates does not faithfully follow the GCC's format for *.gcno
and *.gcda files, and so it will probably not work with other tools
(such as gcov itself) that consume these files.

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