Add freertos_risc_v_chip_specific_extensions.h for 64-bit ports (#385)

* Add freertos_risc_v_chip_specific_extensions.h for 64-bit ports

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

portable/GCC/RISC-V/chip_specific_extensions/RISCV_MTIME_CLINT_no_extensions/freertos_risc_v_chip_specific_extensions.h

@@ -0,0 +1,69 @@
+/*
+ * FreeRTOS Kernel <DEVELOPMENT BRANCH>
+ * Copyright (C) 2021 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * https://www.FreeRTOS.org
+ * https://github.com/FreeRTOS
+ *
+ */
+
+/*
+ * The FreeRTOS kernel's RISC-V port is split between the the code that is
+ * common across all currently supported RISC-V chips (implementations of the
+ * RISC-V ISA), and code that tailors the port to a specific RISC-V chip:
+ *
+ * + FreeRTOS\Source\portable\GCC\RISC-V\portASM.S contains the code that
+ *   is common to all currently supported RISC-V chips.  There is only one
+ *   portASM.S file because the same file is built for all RISC-V target chips.
+ *
+ * + Header files called freertos_risc_v_chip_specific_extensions.h contain the
+ *   code that tailors the FreeRTOS kernel's RISC-V port to a specific RISC-V
+ *   chip.  There are multiple freertos_risc_v_chip_specific_extensions.h files
+ *   as there are multiple RISC-V chip implementations.
+ *
+ * !!!NOTE!!!
+ * TAKE CARE TO INCLUDE THE CORRECT freertos_risc_v_chip_specific_extensions.h
+ * HEADER FILE FOR THE CHIP IN USE.  This is done using the assembler's (not the
+ * compiler's!) include path.  For example, if the chip in use includes a core
+ * local interrupter (CLINT) and does not include any chip specific register
+ * extensions then add the path below to the assembler's include path:
+ * FreeRTOS\Source\portable\GCC\RISC-V\chip_specific_extensions\RISCV_MTIME_CLINT_no_extensions
+ *
+ */
+
+
+#ifndef __FREERTOS_RISC_V_EXTENSIONS_H__
+#define __FREERTOS_RISC_V_EXTENSIONS_H__
+
+#define portasmHAS_SIFIVE_CLINT         1
+#define portasmHAS_MTIME                1
+#define portasmADDITIONAL_CONTEXT_SIZE  0
+
+.macro portasmSAVE_ADDITIONAL_REGISTERS
+    /* No additional registers to save, so this macro does nothing. */
+    .endm
+
+.macro portasmRESTORE_ADDITIONAL_REGISTERS
+    /* No additional registers to restore, so this macro does nothing. */
+    .endm
+
+#endif /* __FREERTOS_RISC_V_EXTENSIONS_H__ */