Add base_isa setting for RISC-V

[romancardenas]

Currently, riscv-rt macros use environment variables to capture characteristics of the target (e.g., RV64 vs RV32, vector table alignment...). While riscv-rt properly sets a few environment variables in itsbuild.rs script for its internals, these environment variables do not outlive the compilation of other crates (e.g., PACs). Thus, PACs, as users of riscv-rt macros, must also define the RISCV_RT_BASE_ISA environment variable to make sure that the generated code supports vectored mode.

This PR includes this field in the settings.yaml file. As ignoring this field may lead to programs hanging, I also included warning messages to recommend that PAC maintainers include this field if their target supports vectored mode.

[burrbull]

Fix errors, please.