[PATCH v2] newlib: fix sys headers installation path

[Alexey Lapshin]

> This patch fixes the header install problem for RTEMS. Why did the
> xtensa port work without this fix? This seems to be a generic issue.


This because of files from "newlib/libc/sys/xtensa/include/" used mostly while newlib build.

While build applications used chip specific core-isa.h file.. E.g. https://github.com/espressif/esp-idf/blob/master/components/xtensa/esp32/include/xtensa/config/core-isa.h

That's the reason why I did not catch the issue.