[CRT] Implement portable + amd64 asm version of fabs/fabsf

Note: older versions of ML64 are broken and don't understand the register form of movq.
See https://stackoverflow.com/questions/24789339/move-quadword-between-xmm-and-general-purpose-register-in-ml64

sdk/lib/crt/math/fabs.c

@@ -0,0 +1,38 @@
+/*
+ * PROJECT:     ReactOS CRT library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Portable implementation of fabs
+ * COPYRIGHT:   Copyright 2021 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+#include <math.h>
+
+#ifdef _MSC_VER
+#pragma function(fabs)
+#endif
+
+_Check_return_
+double
+__cdecl
+fabs(
+    _In_ double x)
+{
+    /* Load the value as uint64 */
+    unsigned long long u64 = *(unsigned long long*)&x;
+
+    /* Clear the sign bit */
+    u64 &= ~(1ULL << 63);
+
+    /* Check for NAN */
+    if (u64 > 0x7FF0000000000000ull)
+    {
+#ifdef _M_IX86
+        /* Set error bit */
+        *(unsigned long long*)&x |= 0x0008000000000000ull;
+#endif
+        return x;
+    }
+
+    /* Convert back to double */
+    return *(double*)&u64;
+}