Pass a0..a2 to main()

[Disasm]

This PR delivers arguments stored in a0..a2 to main.

Fixes #92

Tested with:

• build the empty example for riscv64imac-unknown-none-elf with a memory file which puts everything to 0x80000000
• run qemu-system-riscv64 -nographic -machine virt -bios target/riscv64imac-unknown-none-elf/release/examples/empty -s -S
• connect to qemu with gdb (target remote :1234)
• set breakpoint to main (b main)
• resume execution (continue)
• inspect registers (info registers)

a0             0x0	0
a1             0x87000000	2264924160
a2             0x1028	4136

• check header (x/xw 0x87000000)

0x87000000:	0xedfe0dd0

[SimonSapin]

I’ve tested this in https://github.com/SimonSapin/riscv-qemu-demo, passing the second argument of main to Fdt::from_ptr works. Looks good to me.

[almindor]

lgtm

[almindor]

bors r+

[bors]

Build succeeded:

• ci-linux (1.59.0)
• ci-linux (stable)
• Rustfmt

[almindor]

@Disasm @SimonSapin I forgot, we should add a CHANGELOG entry for this. Could any of you please make a PR for that?

[Disasm]

@almindor indeed, thank you! I created #96 to fix this.