compiler.c.elf.extra_flags is in the wrong place - precompiled libs, ldflags

In platform.txt, the recipe for combining the elf file is incorrect. It should be placed before the linker flags for other static libraries like below.

Background:
In order for the arduino-builder to be able to use precompiled libs, the ldflags are loaded into compiler.c.elf.extra_flags (as per @facchinm 's [commits](https://github.com/arduino/arduino-builder/pull/219) in March 2017).

As per `ld` conventions, one should list the linker flags at the end. This [stackoverflow](https://stackoverflow.com/questions/5005363/undefined-reference-to-sin) issue highlights a case.

```
## Combine gc-sections, archives, and objects
recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" "-L{build.path}" {compiler.c.elf.flags} "-T{build.variant.path}/{build.ldscript}" "-Wl,-Map,{build.path}/{build.project_name}.map" --specs=nano.specs --specs=nosys.specs {compiler.ldflags} -o "{build.path}/{build.project_name}.elf" {object_files} -Wl,--start-group {compiler.arm.cmsis.ldflags} {compiler.c.elf.extra_flags} -lm "{build.path}/{archive_file}" -Wl,--end-group
```

Open platform.txt and inspect the recipe.c.combine.pattern entry for the ELF-linking command. Compare the ordering of compiler.c.elf.extra_flags with the linker flags and static libraries against the command shown in the issue; the work is done when the recipe reflects that ordering for precompiled libraries.

Written by the indexing model from the issue text.