[dynarmic] replace mcl::bit_cast with std::bit_cast; fix IR_emit codepath warnings for UNREACHABLE(); remove type trait mcl::integer_of_size dependency (#2775)

- std::bit_cast already on libc++ (see also #2774)
- type trait is a 5-liner for u8/u16/u32/u64
- UNREACHABLE(); can just be converted into __builtin_unreachable() (TODO: eventually phase out to std::unreachable() once all platforms support it)\
The templatery parametrization is creating many copies of the same function, polluting i-cache and just bloating code size for minimal savings (and in fact could be negative net savings because of the bloated code size) - this is mostly in preparation to a partial removal of template<size_t XXX> in functions to remove instancing (think of the function being copied x4 times... because the compiler is forced to inline it...)
Signed-off-by: lizzie lizzie@eden-emu.dev

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/2775
Reviewed-by: crueter <crueter@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Co-authored-by: lizzie <lizzie@eden-emu.dev>
Co-committed-by: lizzie <lizzie@eden-emu.dev>

src/dynarmic/tests/test_generator.cpp

@@ -17,7 +17,6 @@
 #include <vector>
 
 #include <mcl/bit/swap.hpp>
-#include <mcl/macro/architecture.hpp>
 #include "dynarmic/common/common_types.h"
 
 #include "./A32/testenv.h"