linker: arc: support MWDT-specific input section naming scheme

The MWDT linker has a specific input section sorting mechanism,
designated as x$y. Some prebuilt ARC libraries actually use it,
causing a lot of warnings when the linker decides where to place
their symbols. Provide explicit instructions in the linker script
instead of making the linker guess.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>