[EABI] RV32 only stack support instructions

[jnk0le]

Due to 2xXLEN stack alignment, RV32 eabi needs extra compressed instructions to efficiently operate on 8 byte aligned stack, otherwise non compressed instructions will have to be used when limiting stack waste is important.

• push/pop with 8 byte stack granularity (probably 3rd imm bit?)- most obvious alternative is to not use push/pop, or use it for 3-4 reg chunks only and handle rest by addi + loads/stores

• C(M).ADDI8SP - probably not necessary, push/pop with 8 byte granularity can prepare initial 8 byte allocation for an following addi16sp. Non zcmpe EABI cores may need it.

[tariqkurd-repo]

why 8-byte aligned? Is this in the EABI proposal?

[jnk0le]

yes https://github.com/riscv-non-isa/riscv-eabi-spec/blob/master/EABI.adoc#4-eabi-stack-alignment

currently 2xXLEN for both RV32 and RV64