Replace per-op save/restore of FPCR in EmitWithVmxFpcr with tracked
lazy switching via ChangeFpcrMode. Consecutive VMX float ops in the
same basic block now emit a single FPCR switch instead of one per op.
ChangeFpcrMode loads pre-computed values from the backend context
(fpcr_fpu/fpcr_vmx) instead of MRS + read-modify-write, eliminating
an expensive system register read per switch.
FPU mode is restored at block boundaries and calls via ForgetFpcrMode.
Scalar FP sequences guard with ChangeFpcrMode(Fpu) which is a no-op
when already in FPU mode.
Adds `platform_arm64` similar to `platform_arm64` to populate a 64-bit feature-flag value and creates a `a64_extension_mask` configuration-mask for the detection and utilization of certain arm ISA features. The first feature being `FEAT_LSE` which maps to `XBYAK_AARCH64_HWCAP_ATOMIC`.
Just like the x64 backend, implements `IsFeatureEnabled` to the emitter for checking if the host processor supports a certain subset of feature flag(s) at once.
Optimizes `OPCODE_ATOMIC_EXCHANGE`, `OPCODE_ATOMIC_COMPARE_EXCHANGE`,
and `OPCODE_RESERVED_STORE` with `FEAT_LSE` instructions.
Expands the vector register allocator from 20 to 28 registers by
including v8-v15. The HostToGuestThunk now saves/restores full Q8-Q15
(128-bit) instead of just D8-D15, and the GuestToHostThunk adds Q8-Q15
to its save/restore set since AAPCS64 only preserves their lower 64
bits across C calls.
Reduces register pressure for VMX-heavy guest code at the cost of
slightly larger thunk stack frames.
Reserves x19 to permanently hold the A64BackendContext pointer
(x20 - sizeof(A64BackendContext)), set once in the HostToGuestThunk.
Since x19 is AAPCS64 callee-saved, it survives all host calls for free.
Eliminates repeated 2-instruction mov+sub sequences in stackpoint
push/pop, stack synchronization, FPCR management, and reserved
load/store paths.
Reduces allocatable GPRs from 8 to 7 (like on x64).
Based entirely off existing xbyak x86 implementation and available
tests. Still needs a lot of optimization and testing on non-Windows
platforms.
So far passes all tests and boots at least some games on Windows.
