riscv64 infinite recursing during multiplication __muldi3

[johannst]

When I compile a multiplication on rv64 without the m extension I'll end up in an infinite recursion as:

#2  0x0000000000021318 in core::num::{impl#8}::wrapping_mul (self=0, rhs=257)
    at /home/johannst/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:1249
#3  compiler_builtins::int::{impl#21}::zero_widen_mul (self=0, rhs=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mod.rs:337
#4  0x00000000000218d8 in compiler_builtins::int::mul::Mul::mul<u64> (self=0, rhs=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mul.rs:16
#5  0x0000000000021ea0 in compiler_builtins::int::mul::__muldi3 (a=0, b=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mul.rs:104
#6  0x000000000002177c in compiler_builtins::int::mul::__muldi3::__muldi3 (a=0, b=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/macros.rs:336

#7  0x0000000000021318 in core::num::{impl#8}::wrapping_mul (self=0, rhs=257)
    at /home/johannst/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:1249
#8  compiler_builtins::int::{impl#21}::zero_widen_mul (self=0, rhs=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mod.rs:337
#9  0x00000000000218d8 in compiler_builtins::int::mul::Mul::mul<u64> (self=0, rhs=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mul.rs:16
#10 0x0000000000021ea0 in compiler_builtins::int::mul::__muldi3 (a=0, b=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/int/mul.rs:104
#11 0x000000000002177c in compiler_builtins::int::mul::__muldi3::__muldi3 (a=0, b=257)
    at /home/johannst/.cargo/registry/src/git.colasdn.top-1ecc6299db9ec823/compiler_builtins-0.1.71/src/macros.rs:336
...

A potential solution would be to implement the __muldi3 intrinsic for rv64 similar to __mulsi3 for rv32.

To unblock myself I created following patch rv64-muldi3 - b945767, if that would be an acceptable starting point I am happy to create a PR :)

[AaronKutch]

Looks good, I would go ahead and make the PR