[sw] rework intrinsics (e.g. for custom instructions)

[stnolting]

This PR is a rework of the "intrinsics" library (set of macros), which can be used to implement custom instructions, and also of the two applications that use these intrinsics (the FPU test program and the bit-manipulation test program).

The new intrinsics are far more convenient to handle as they can be used as plain C function without the need to pay attention to register allocation or any other background assembly issues (= intrinsics).

The following example uses a R2-format intrinsic to rebuild the add instruction as custom instruction:

uint32_t rs1 = 0x12345678;
uint32_t rs2 = some_function();
uint32_t res = CUSTOM_INSTR_R2_TYPE(0b0000000, rs2, rs1, 0b000, 0b0110011);

Thanks to @google for their CFU-Playground project, which provided the idea for the improved intrinsic macro library and also thanks to @umarcor for pointing that out to me.