linker: allow SoC to insert linker script fragments

This allows the SoC to specify some additional linker script
fragments into the bss, data and read-only data sections.

For example, the Cypress PSOC6 has a few input sections that
must be put into bss and data sections. Without specifying
these in the linker script, they are consider orphan sections
and the placement is based on linker heuristic which is
arbitrary.

POSIX is not supported as the main linker script is
provided by the host system's binutils and we have no control
over it. Also, currently Xtensa SoCs have their own linker
scripts so there is no need to this feature.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>