userland: add assembly support

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

userland/arch/aarch64/floating_point.S

@@ -0,0 +1,60 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#advanced-simd-instructions */
+
+#include "common.h"
+
+ENTRY
+    /* 1.5 + 2.5 == 4.0
+     * using 64-bit double immediates.
+     */
+    fmov d0, 1.5
+    fmov d1, 2.5
+    fadd d2, d0, d1
+    fmov d3, 4.0
+    /* Unlike VFP vcmp, this stores the status
+     * automatically in the main CPSR.
+     */
+    fcmp d2, d3
+    ASSERT(beq)
+
+    /* Now with a memory stored value. */
+.data
+my_double_0:
+    .double 1.5
+my_double_1:
+    .double 2.5
+my_double_sum_expect:
+    .double 4.0
+.text
+    ldr d0, my_double_0
+    ldr d1, my_double_1
+    fadd d2, d0, d1
+    ldr d3, my_double_sum_expect
+    fcmp d2, d3
+    ASSERT(beq)
+
+    /* Now in 32-bit. */
+    fmov s0, 1.5
+    fmov s1, 2.5
+    fadd s2, s0, s1
+    fmov s3, 4.0
+    fcmp s2, s3
+    ASSERT(beq)
+
+    /* TODO why? What's the point of q then?
+     * Error: operand mismatch -- `fmov q0,1.5'
+     */
+#if 0
+    fmov q0, 1.5
+#endif
+
+    /* Much like integers, immediates are constrained to
+     * fit in 32-byte instructions. TODO exact rules.
+     *
+     * Assembly here would fail with:
+     *
+     * Error: invalid floating-point constant at operand 2
+     */
+#if 0
+    fmov d0, 1.23456798
+#endif
+EXIT