Enable precompiled libraries (#4209)

The latest versions of Arduino IDE shifted the responsibility for precompiled libraries support to the core developers, which breaks precompiled library support in esp32 Arduino core. See arduino/ArduinoCore-avr#52 for more details:

```
In this new version of the builder we are not doing any heuristics to find the right spot where the ldflags should be inserted (this was causing many bugs on its own); instead, we fully trust the core makers to add explicit support to precompiled libs.
```

This chage re-enables precompiled library support in the esp32 Arduino core.

platform.txt

@@ -44,6 +44,8 @@ compiler.ar.flags=cru
 
 compiler.size.cmd=xtensa-esp32-elf-size
 
+compiler.libraries.ldflags=
+
 # This can be overriden in boards.txt
 build.flash_size=4MB
 build.flash_mode=dio
@@ -82,7 +84,7 @@ recipe.S.o.pattern="{compiler.path}{compiler.c.cmd}" {compiler.cpreprocessor.fla
 recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
 
 ## Combine gc-sections, archives, and objects
-recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} {compiler.c.elf.extra_flags} -Wl,--start-group {object_files} "{archive_file_path}" {compiler.c.elf.libs} {compiler.libraries.ldflags} -Wl,--end-group -Wl,-EL -o "{build.path}/{build.project_name}.elf"
+recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} {compiler.c.elf.extra_flags} {compiler.libraries.ldflags} -Wl,--start-group {object_files} "{archive_file_path}" {compiler.c.elf.libs} -Wl,--end-group -Wl,-EL -o "{build.path}/{build.project_name}.elf"
 
 ## Create eeprom
 recipe.objcopy.eep.pattern={tools.gen_esp32part.cmd} -q "{build.path}/partitions.csv" "{build.path}/{build.project_name}.partitions.bin"