userland: add assembly support

Move arm assembly cheat here, and start some work on x86 cheat as well.

userland/arch/arm/c/inc_float.c

@@ -0,0 +1,28 @@
+/* https://stackoverflow.com/questions/53960240/armv8-floating-point-output-inline-assembly */
+
+#include <assert.h>
+
+int main(void) {
+    float my_float = 1.5;
+    __asm__ (
+        "vmov s0, 1.0;"
+        "vadd.f32 %[my_float], %[my_float], s0;"
+        : [my_float] "+t" (my_float)
+        :
+        : "s0"
+    );
+    assert(my_float == 2.5);
+
+    /* Undocumented %P
+     * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89482
+     */
+    double my_double = 1.5;
+    __asm__ (
+        "vmov.f64 d0, 1.0;"
+        "vadd.f64 %P[my_double], %P[my_double], d0;"
+        : [my_double] "+w" (my_double)
+        :
+        : "d0"
+    );
+    assert(my_double == 2.5);
+}