[rom] Remove symbol for chip_info_size and turn it into an autogenerated #define

#29577 uses a linker symbol to know the chip info offset from the end of the ROM but this can cause issues with some compilers. Since this offset is fixed and in an autogenerated file, we should also autogenerate a `#define` with the same value and remove this linker symbol.

_Originally posted by @pamaury in https://github.com/lowRISC/opentitan/issues/29577#issuecomment-4188827370_

Start by reviewing issue #29577 and locating the autogenerated file and linker definition for chip_info_size. Confirm how the fixed chip-info offset is generated, then ensure the autogenerated #define carries the same value and the linker symbol is removed; done means the ROM build still produces the expected offset without compiler issues.

Written by the indexing model from the issue text.