Add initial crt0 for Ventus OpenCL kernel.

compiler-rt/lib/ventus/crt0.S

@@ -0,0 +1,44 @@
+/**
+ * Copyright (c) 2023 Terapines Technology (Wuhan) Co., Ltd
+ *
+ * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+ * See https://llvm.org/LICENSE.txt for license information.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+/**	
+ * crt0.S : Entry point for Ventus OpenCL C kernel programs
+ */
+
+  .text
+  .global _start
+  .type   _start, @function
+_start:
+  # set global pointer register
+  .option push
+  .option norelax
+  la gp, __global_pointer
+  .option pop
+
+  # allocate warp and per-thread level stack pointers
+  la sp, __stack_pointer # sp points to local memory
+  la tp, __thread_stack_pointer # tp points to private memory
+
+  # clear BSS segment
+  la      a0, _edata
+  la      a2, _end
+  beq     a0, a2, 2f
+1:
+  sw      zero, (a0)
+  addi    a0, a0, 4
+  bltu    a0, a2, 1b
+
+2:
+  csrr a3, CSR_KNL	  # get addr of kernel metadata
+  lw a2, 0(a3)            # get kernel program address
+  lw a0, 4(a3)            # get kernel 1st arg address
+  jalr a2                 # call kernel program
+
+  # call exit routine
+  # tail    exit
+.size  _start, .-_start