interrupt: Various improvements

[taiki-e]

• Support RISC-V supervisor mode under portable_atomic_s_mode cfg

  On RISC-V without A-extension, this generates code for machine-mode (M-mode) by default. If you pass the --cfg portable_atomic_s_mode together, this generates code for supervisor-mode (S-mode). In particular, qemu-system-riscv* uses OpenSBI as the default firmware.

• Support disabling FIQs on pre-v6 ARM under portable_atomic_disable_fiq cfg

  On pre-v6 ARM, this disables only IRQs by default. For many systems (e.g., GBA) this is enough. If the system need to disable both IRQs and FIQs, you need to pass the --cfg portable_atomic_disable_fiq together.

• Interrupt-related documentation improvements

  • README.md
  • src/imp/interrupt/README.md

• Defer mask until just before branch

  This does not change the code generation, but in the actual generated code the mask is deferred until just before the branch, like this. Since there has been some misleading discussion about this in the past, we will use code that more closely matches the generated code.

  For MSP430 and AVR, it will be done in interrupt: Optimize restore on AVR and MSP430 #40.

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci
• test (aarch64-apple-darwin)
• test (aarch64-unknown-linux-gnu)
• valgrind (aarch64-unknown-linux-gnu)