GitLab

General change:
 - library compilation bails out if the precompiled object is found.
   This allows mixed libraries that fallback using sources if no suitable precompiled object is found

ARM float specification change:
 - Cortex M4+ allows specifying different flags for floating point ABI
   This patch introduces a second level of subfolder that MUST be used is -mfpu or -mfloat-abi are specified on the commandline
   Since there's no clear specification (unlike build.mcu), the values are extracted from c++ recipe

   For example, for a target which commandline contains `-mfloat-abi=hard -mfpu=fpv4-sp-d16` , the precompiled search path will be
   `$libfolder/cortex-m4/fpv4-sp-d16-hard`
   If that folder doesn't exist the library will be compiled from sources

Fixes https://github.com/arduino/arduino-builder/issues/256