Constify error_desc and perror

[visitorckw]

The error_desc array and the perror pointer are never modified after initialization. Marking them as const moves the data to the read-only section, improving safety.

Before:
$ riscv-none-elf-size ./build/kernel/error.o
text data bss dec hex filename
398 172 0 570 23a ./build/kernel/error.o

After:
$ riscv-none-elf-size ./build/kernel/error.o
text data bss dec hex filename
570 0 0 570 23a ./build/kernel/error.o

[jserv]

Thank @visitorckw for contributing!