add tinygo device files

targets/device/riscv/start.S

@@ -0,0 +1,59 @@
+.section .init
+.global _start
+.type _start,@function
+
+_start:
+    // If we're on a multicore system, we need to wait for hart 0 to wake us up.
+#if TINYGO_CORES > 1
+    csrr a0, mhartid
+
+    // Hart 0 stack
+    bnez a0, 1f
+    la sp,      _stack_top
+
+1:
+    // Hart 1 stack
+    li a1, 1
+    bne a0, a1, 2f
+    la sp,      _stack1_top
+
+2:
+    // Hart 2 stack
+    #if TINYGO_CORES >= 3
+    li a1, 2
+    bne a0, a1, 3f
+    la sp,      _stack2_top
+    #endif
+
+3:
+    // Hart 3 stack
+    #if TINYGO_CORES >= 4
+    li a1, 3
+    bne a0, a1, 4f
+    la sp,      _stack3_top
+    #endif
+
+4:
+    // done
+
+#if TINYGO_CORES > 4
+#error only up to 4 cores are supported at the moment!
+#endif
+
+#else
+    // Load the stack pointer.
+    la sp,      _stack_top
+#endif
+
+    // Load the globals pointer. The program will load pointers relative to this
+    // register, so it must be set to the right value on startup.
+    // See: https://gnu-mcu-eclipse.github.io/arch/riscv/programmer/#the-gp-global-pointer-register
+    // Linker relaxations must be disabled to avoid the initialization beign
+    // relaxed with an uninitialized global pointer: mv gp, gp
+    .option push
+    .option norelax
+    la gp,      __global_pointer$
+    .option pop
+
+    // Jump to runtime.main
+    call main