Fix panic due to overflow in riscv.rs and int/shift.rs

[taiki-e]

I ran into this while testing riscv32i-unknown-none-elf with -Z build-std=core,alloc.

panicked at 'attempt to add with overflow', /Users/taiki/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.87/src/riscv.rs:42:17

(Can be reproduced by cloning semihosting repo, comment out this line, and run ./tools/no-std riscv32i-unknown-none-elf)

[taiki-e]

I found another panic due to overflow on the other targets (thumbv5te-none-eabi) as well. I will update the PR to include a fix for them later.

panicked at 'attempt to subtract with overflow', /Users/taiki/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.87/src/int/shift.rs:15:39

panicked at 'attempt to subtract with overflow', /Users/taiki/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.87/src/int/shift.rs:60:69

(Can be reproduced by cloning portable-atomic repo, comment out this line, and run ./tools/no-std thumbv5te-none-eabi)

[davidben]

A related issue we uncovered when we hit a variation on this in Arm: llvm/llvm-project#62743