[PATCH] Enable riscv-elf targets

Hi,

I've been struggling to make a RISC-V cross compiler. This `manifest.scm`

file failed to create because there was no available dynamic linker for

the targets which, in the end, don't need any dynamic linker:

manifest.scm:

``` scm

(use-modules (gnu packages cross-base)

(gnu packages embedded)

(gnu packages man))

(packages->manifest

`(,@(let* ((triplet "riscv32-unknown-elf")

(binutils (cross-binutils triplet)))

(list

binutils

(cross-gcc triplet

#:xbinutils binutils

#:libc #f)))

,man-db))

```

I realized there were some targets missing (and there are more!) in the

bootstrap.scm file, so I think adding them is harmless and lets us

make cross compilers that are not available otherwise.

I'd like to add riscv32-linux too, but that involves setting the correct

path and I don't know it.

Hope this patch is helpful.

From 58785556b7448cfe32916d3842bce398b360c2de Mon Sep 17 00:00:00 2001

* gnu/packages/bootstrap.scm: (glibc-dynamic-linker): Set

riscv64-*-elf targets to "no-ld"

---

gnu/packages/bootstrap.scm | 2 ++

1 file changed, 2 insertions(+)

--

2.29.2

This issue from RISC-V GNU Toolchain has more information

related with the issue described:

For more information on the dynamic linker names I didn't know:

lp32, ilp32f and ilp32d are the ABI.

i -> integer

l -> long

p -> pointer

f -> float

d -> double

The chars in the beginning indicate what kind of data is sent to 32 bit registers.

The suffix says which ones are sent to floating point registers.

So in ilp32 floating point numbers are sent to integer registers, while in ilp32d double precision numbers are sent to floating point registers.

Not every ABI is compatible with every single RISC-V configuration. RV32I for instance, doesn't support floating point numbers (they are supported in the F extension) so it doesn't provide floating point registers meaning that it's not able to use any ABI that has a `d` or an `f` as suffix.

But RISC-V configurations with floating point modules can actually run the simplest ABI if they want so we probably need to add many entries here :)

Does this make sense?