Add a workaround for gcov-based coverage with clang

- Allow overriding the gcov tool with the GCOV env variable in cc_configure
- Symlink the GCOV tool in collect-coverage.sh to a temporary location with
  the name "gcov"

This allows the user to specify GCOV=llvm-cov in the environment of a bazel
build, which then leads to cc_configure picking up llvm-cov in the crosstool,
which the collect-coverage.sh script then uses as "gcov".

On linux distributions, the gcov tool does not generally work with clang
coverage output, so this provides at least a workaround for running coverage
with clang (by setting the GCOV env variable).

Closes #5040.

PiperOrigin-RevId: 193487773

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