Add libc_musl as a dependency of RuleBuilderCommand.BuiltTool

Assume calls to RuleBuilderCommand.BuiltTool may refer to a tool
that was built against musl libc, and add it as a dependency
so that it is copied into the sandbox.  This emulates the
behavior of compiling against glibc, which is available from the
host sysroot when running in the sandbox.

Bug: 190084016
Test: m USE_HOST_MUSL=true sdk-repo-build-tools
Change-Id: Ieafdcceb818f9c31595487aab3ffbafba1412b3a