test(toolchain): use flag_values in test platform defs

[aignas]

This PR uses the flag_values from the platform definitions
for the toolchains so that in the future we can distinguish
between musl and glibc toolchains in our tests.

For now the change is no-op.

As part of this change we are also registering the coverage
tools so that we can run bazel coverage with no errors.

See comment on #2095.

[rickeylev]

Can you also update the description to better describe the change? A PR/issue number is nice, but I prefer commits to be more self-descriptive.