LibC: Separate arch dependent fenv functions

Remove all functions with platform #if's from fenv, and
add arch dependent implementations instead. The build
system now selects the implementation based on the platform.

Userland/Libraries/LibC/arch/riscv64/fenv.cpp

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2024, Tom Finet <tom.codeninja@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Types.h>
+#include <fenv.h>
+
+extern "C" {
+
+int fegetenv(fenv_t* env)
+{
+    if (!env)
+        return 1;
+
+    (void)env;
+    TODO_RISCV64();
+    return 0;
+}
+
+int fesetenv(fenv_t const* env)
+{
+    if (!env)
+        return 1;
+
+    (void)env;
+    TODO_RISCV64();
+    return 0;
+}
+
+int feholdexcept(fenv_t* env)
+{
+    fegetenv(env);
+
+    fenv_t current_env;
+    fegetenv(&current_env);
+
+    (void)env;
+    TODO_RISCV64();
+
+    fesetenv(&current_env);
+    return 0;
+}
+
+int fesetexceptflag(fexcept_t const* except, int exceptions)
+{
+    if (!except)
+        return 1;
+
+    fenv_t current_env;
+    fegetenv(&current_env);
+
+    exceptions &= FE_ALL_EXCEPT;
+
+    (void)exceptions;
+    (void)except;
+    TODO_RISCV64();
+
+    fesetenv(&current_env);
+    return 0;
+}
+
+int fegetround()
+{
+    TODO_RISCV64();
+}
+
+int fesetround(int rounding_mode)
+{
+    if (rounding_mode < FE_TONEAREST || rounding_mode > FE_TOWARDZERO)
+        return 1;
+
+    TODO_RISCV64();
+    return 0;
+}
+
+int feclearexcept(int exceptions)
+{
+    exceptions &= FE_ALL_EXCEPT;
+
+    fenv_t current_env;
+    fegetenv(&current_env);
+
+    (void)exceptions;
+    TODO_RISCV64();
+
+    fesetenv(&current_env);
+    return 0;
+}
+
+int fetestexcept(int exceptions)
+{
+    (void)exceptions;
+    TODO_RISCV64();
+}
+
+int feraiseexcept(int exceptions)
+{
+    fenv_t env;
+    fegetenv(&env);
+
+    exceptions &= FE_ALL_EXCEPT;
+
+    (void)exceptions;
+    TODO_RISCV64();
+}
+}