Initial RISC-V support

[athei]

This makes ink! compile on RISC-V. There are two things which are just filled with interim values:

• Location and size of the heap
• Function ids and ABI of the host functions

Both of these things are not final and will be replaced once we have written the specification of our VM. Nonetheless we should still merge the code once this PR is in a good state: This allows us to iterate with the new RISC-V VM and prevent RISC-V from breaking in the future.

I will also need to make changes to cargo contract. As of right now I am compiling manually. Once a new cargo contract version is released I can add the CI jobs to this PR. Those will replace our no-std stop gap CI jobs.

Notable changes required for support:

• RISC-V does not support cdylib. Instead, we build contracts as bin target (to be added by cargo contract). We need to use a single entry point in case of RISC-V. This is also why every contract needs to add no_main in addition to no_std when building for RISC-V (wasm doesn't care). Wasm can have multiple entry points even when build as bin. So we can do that unconditionally.
• Panic handler needs support for RISC-V
• Memory allocator needs support for RISC-V
• oom handler needs support for RISC-V. I opted for removing the handler and raising the Rust version. A default handler is supplied starting with version 1.68. Hence we no longer need the unstable feature.

[athei]

Apparently doc tests are not run if a library doesn't include the rlibtarget. By adding it to all integration tests (by falling back to default) the multisig doc tests were suddenly run. I fixed them up.

Also needed to update the dyling toolchain as we no require 1.68.

[codecov-commenter]

Codecov Report

Merging #1718 (412bffa) into master (3f11e0e) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1718      +/-   ##
==========================================
- Coverage   70.74%   70.73%   -0.02%     
==========================================
  Files         205      205              
  Lines        6478     6479       +1     
==========================================
  Hits         4583     4583              
- Misses       1895     1896       +1     

Impacted Files	Coverage Δ
crates/ink/codegen/src/generator/dispatch.rs	92.95% <ø> (ø)
crates/allocator/src/bump.rs	86.61% <66.66%> (-3.47%)	⬇️

... and 3 files with indirect coverage changes

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[athei]

Ready for review. Let's get this in as-is because it touches a lot of files and I expect merge conflicts. I will do a follow up PR where I will use cargo contract in the CI.

[athei]

The code for riscv uses allocation to provide a buffer to encode the arguments for the host into. This will be changed to a static buffer in a follow up. It requires a bigger refactor.