System preprocessor definitions for architecture & ABI

[ilg-ul]

(this should be passed to the team that will implement the compiler, but for reference I enter it here)

The current ABI selection must be available via a compiler system preprocessor definition, so the software will know how to handle exception frames (and possible other low level use cases).

The only document where I saw some preprocessor definitions is:

https://github.com/riscv/riscv-toolchain-conventions

but it does not include a macro for the ABI.

Neither the dumps that I did some time ago do not list the ABI:

https://gnu-mcu-eclipse.github.io/arch/riscv/programmer/#preprocessor-macros

[aswaterman]

The UABI defines __riscv_float_abi_soft, __riscv_float_abi_single, or __riscv_float_abi_double to indicate which of the three UABI calling conventions is in use for a given XLEN. I think the EABI should follow the same convention for simplicity, but additionally define __riscv_eabi to distinguish the EABI from the UABI.