linker: align _ramfunc_ram/rom_start/size linker symbol names

Cleanup and preparation commit for linker script generator.

Zephyr linker scripts provides start and end symbols for each section,
and sometimes even size and LMA start symbols.

Generally, start and end symbols uses the following pattern, as:
Section name:             foo
Section start symbol:     __foo_start
Section end symbol:       __foo_end

However, this pattern is not followed consistently.
To allow for linker script generation and ensure consistent naming of
symbols then the following pattern is introduced consistently to allow
for cleaner linker script generation.
Section name:             foo
Section start symbol:     __foo_start
Section end symbol:       __foo_end
Section size symbol:      __foo_size
Section LMA start symbol: __foo_load_start

This commit aligns the symbols for _ramfunc_ram/rom to other symbols and
in such a way they follow consistent pattern which allows for linker
script and scatter file generation.

The symbols are named according to the section name they describe.
Section name is `ramfunc`

The following symbols are aligned in this commit:
-  _ramfunc_ram_start  -> __ramfunc_start
-  _ramfunc_ram_end    -> __ramfunc_end
-  _ramfunc_ram_size   -> __ramfunc_size
-  _ramfunc_rom_start  -> __ramfunc_load_start

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>