Add inline asm support for amdgpu

Add support for inline assembly for the amdgpu backend (the
amdgcn-amd-amdhsa target).
Add register classes for `vgpr` (vector general purpose register) and
`sgpr` (scalar general purpose register).
The LLVM backend supports two more classes, `reg`, which is either VGPR
or SGPR, up to the compiler to decide. As instructions often rely on a
register being either a VGPR or SGPR for the assembly to be valid, reg
doesn’t seem that useful (I struggled to write correct tests for it), so
I didn’t end up adding it.
The fourth register class is AGPRs, which only exist on some hardware
versions (not the consumer ones) and they have restricted ways to write
and read from them, which makes it hard to write a Rust variable into
them. They could be used inside assembly blocks, but I didn’t add them
as Rust register class.

There are a few change affecting general inline assembly code, that is
`InlineAsmReg::name()` now returns a `Cow` instead of a `&'static str`.
Because amdgpu has many registers, 256 VGPRs plus combinations of 2 or 4
VGPRs, and I didn’t want to list hundreds of static strings, the amdgpu
reg stores the register number(s) and a non-static String is generated
at runtime for the register name.
Similar for register classes and supported_types.

Vectors of 64-bit types are supported by the LLVM backend, but omitted
here to make the code simpler. There is currently no systematic support
in LLVM of which vectors of 64-bit types are supported. Also, they are
likely seldomly unused, vectors of 16- and 32-bit types are important.