treewide: lib/stack: Make base_address arch specific

Calculating the offset of an address is image specific, which is
architecture specific. Until now, all architectures and architecture
configurations which select CONFIG_RELOC were able to subtract
_etext, but the EFI configuration of riscv cannot (it must subtract
ImageBase). Make this function weak, such that an architecture may
override it when necessary, to accommodate the image layout. Then,
immediately supply the riscv override.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>