[feature request] add a way to add ldflags even without precompiled libraries

[nerdralph]

Bug Report

library.properties ldflags is ignored unless precompiled=true

Current behavior

After failing to get ldflags to work, I checked the source and found it is only used with precompiled libraries.

arduino-cli/legacy/builder/phases/libraries_builder.go

Line 137 in 1c7920f

if library.Precompiled {

Expected behavior

ldflags, when specified, should always get added to the link line.

Environment

arduino-cli.exe Version: 0.10.0 Commit: ec5c3ed
Win7e/mingw32

Additional context

I'm writing a bootloader library, and was trying to use the --section-start linker option.
While reviewing the code, I also noticed there is an undocumented precompiled=full option for library.properties.