[sw] Modify flash_link.ld for llvm coverage
[sw] Modify flash_link.ld for llvm coverage

When code is compiled with coverage enabled, the compiler emits various
symbols and sections that are used at runtime to collect coverage data.
This change modifies flash_link.ld linker script to preserve these
sections.

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

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