riscv: Fix C++ exception handling info linking

The RISC-V architecture linker script was including `cplusplus-ram.ld`
linker script before `__data_region_start`, and this caused the content
of `.gcc_except_table` section to be not copied to the RAM by the
`z_data_copy` function; leading to the C++ exception handling
malfunction.

This commit relocates the `cplusplus-ram.ld` linker script inclusion
such that the contents of the relevant sections are properly copied by
the `z_data_copy` function.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>