Overcome longdouble_soft problem on WoA64 hosts

That 'soft' implementation (of an 80-bit x87 value used for compile-time
reals on Windows MSVC x86[_64] hosts, if their native `real` is a 64-bit
double with an LDC host compiler) is still based on x87 inline asm,
so cannot be used in a WoA64 compiler binary.

So use a 64-bit native `real` for compile-time reals on WoA64 hosts.

Author: kinke

dmd/main.d

@@ -442,8 +442,11 @@ else
     reconcileLinkRunLib(params, files.length, target.obj_ext);
     version(CRuntime_Microsoft)
     {
-        import dmd.root.longdouble;
-        initFPU();
+        version(AArch64) { /* no longdouble_soft support on arm64 host */ } else
+        {
+            import dmd.root.longdouble;
+            initFPU();
+        }
     }
     import dmd.root.ctfloat : CTFloat;
     CTFloat.initialize();

dmd/root/ctfloat.d

@@ -27,13 +27,17 @@ private
 {
     version(CRuntime_DigitalMars) __gshared extern (C) extern const(char)* __locale_decpoint;
 
-    version(CRuntime_Microsoft) extern (C++)
+    version(CRuntime_Microsoft)
     {
-        public import dmd.root.longdouble : longdouble_soft, ld_sprint;
+        version(AArch64) { /* 64-bit real_t */ } else
+        {
+            version = MSVC_X87;
+            import dmd.root.longdouble : longdouble_soft, ld_sprint;
 version (IN_LLVM) {} else
 {
-        import dmd.root.strtold;
+            import dmd.root.strtold;
 }
+        }
     }
 }
 
@@ -193,7 +197,7 @@ extern (C++) struct CTFloat
     // the implementation of longdouble for MSVC is a struct, so mangling
     //  doesn't match with the C++ header.
     // add a wrapper just for isSNaN as this is the only function called from C++
-    version(CRuntime_Microsoft) static if (is(real_t == real))
+    version(MSVC_X87) static if (is(real_t == real))
         pure @trusted
         static bool isSNaN(longdouble_soft ld)
         {
@@ -239,7 +243,7 @@ extern (C++) struct CTFloat
     @system
     static int sprint(char* str, size_t size, char fmt, real_t x)
     {
-        version(CRuntime_Microsoft)
+        version(MSVC_X87)
         {
             auto len = cast(int) ld_sprint(str, size, fmt, longdouble_soft(x));
         }

dmd/root/longdouble.d

@@ -13,7 +13,9 @@ module dmd.root.longdouble;
 
 version (CRuntime_Microsoft)
 {
-    static if (real.sizeof > 8)
+    version (AArch64)
+        alias longdouble = real; // 64-bit
+    else static if (real.sizeof > 8)
         alias longdouble = real;
     else
         alias longdouble = longdouble_soft;
@@ -24,6 +26,7 @@ else
 // longdouble_soft needed when building the backend with
 // Visual C or the frontend with LDC on Windows
 version (CRuntime_Microsoft):
+version (AArch64) { /* cannot use x87 inline asm on arm64 host */ } else:
 extern (C++):
 nothrow:
 @nogc: