Strip ROM symbols

[davydnorris]

Quite a few library functions in libgcc.a and libc.a exist in the chip ROM. Previously the duplicate functions in the esp-open-sdk built libgcc were masked by Espressif supplying their own stripped libgcc.a in the SDK, however the functions in the built libc would always have ended up being used over the ROM versions. This would lead to larger and potentially slower code and possibly even memory allocation differences. Since quite a few core string manipulation functions are some of the ones in ROM, this could be significant.

Espressif has recently added an ar script (based on my libgcc script, but with corrected set of object files) into the SDK. This change adds them to the Makefile