Implement ArmFPURoundMode.cpp

Fixes https://bugs.dolphin-emu.org/issues/12388. Might also fix
other games that have problems with float/paired instructions
in JitArm64, but I haven't tested any.

Source/Core/Common/ArmCPUDetect.cpp

@@ -69,6 +69,7 @@ void CPUInfo::Detect()
   CPU64bit = true;
   Mode64bit = true;
   vendor = CPUVendor::ARM;
+  bFlushToZero = true;
 
 #ifdef _WIN32
   num_cores = std::thread::hardware_concurrency();