Specify -march and -mabi in README.md example

[MarekVCodasip]

Some toolchains (notably the The xPack GNU RISC-V Embedded GCC) build 32 bit binaries by default, spike runs 64 by default, causing an error. This pull request specifies needed parameters to avoid this problem in the example.

[aswaterman]

This one cuts both ways. Some toolchains don't include multilibs for rv64imac/lp64, so this may break elsewhere. I would think that using riscv64-unknown-elf-gcc with no args is more likely to work for more people, and really should be producing RV64 code (riscv32-unknown-elf-gcc would seem like the right name for a compiler that defaults to RV32 code).

[MarekVCodasip]

So in that case, would adding a note about it suffice? Something along the lines of:

NOTE: Depending on your toolchain, it might build 32 bit binaries by default, add -march=rv64imac -mabi=lp64 to GCC to prevent this.

[aswaterman]

Yeah, seems reasonable.