Support LLVM 9.0

[XVilka]

LLVM 9.0 was recently released, with many things inside:

• The RISCV target is no longer “experimental” (see Changes to the RISCV Target below for more details).
• Support for target-independent hardware loops in IR has been added, with PowerPC and Arm implementations.
• Various optimisations

Changes to the LLVM IR

• Added immarg parameter attribute. This indicates an intrinsic parameter is required to be a simple constant. This annotation must be accurate to avoid possible miscompiles.
• The 2-field form of global variables @llvm.global_ctors and @llvm.global_dtors has been deleted. The third field of their element type is now mandatory. Specify i8* null to migrate from the obsoleted 2-field form.
• The byval attribute can now take a type parameter: byval(<ty>). If present it must be identical to the argument’s pointee type. In the next release we intend to make this parameter mandatory in preparation for opaque pointer types.
• atomicrmw xchg now allows floating point types
• atomicrmw now supports fadd and fsub

Depends on https://github.com/klee/klee/issues/1136