gen/target: make real 128bit on RISC-V targets

[ljmf00]

RISC-V ILP32/LP64 ABI defines long double as 128bit.

Reference: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#cc-type-details
Signed-off-by: Luís Ferreira contact@lsferreira.net

[kinke]

I took the liberty of fixing this for LLVM < 9, refactoring the function while at it. The switch can be replaced by a terser if (triple.isX86()) … else if … chain once support for LLVM < 9 is dropped.

[ljmf00]

The ultimate goal would be to use clang::TargetInfo, which gives you information about all target-specific types, sizes, etc... I didn't search in which version this is available, but I think the downside is the dependency to libclang. But if you do it statically, maybe it is not a big of an hassle.