add support for linux_riscv64 and freestanding_riscv64

base/runtime/entry_unix.odin

@@ -34,6 +34,9 @@ when ODIN_BUILD_MODE == .Dynamic {
 		} else when ODIN_OS == .Darwin && ODIN_ARCH == .arm64 {
 			@require foreign import entry "entry_unix_no_crt_darwin_arm64.asm"
 			SYS_exit :: 1
+		} else when ODIN_ARCH == .riscv64 {
+			@require foreign import entry "entry_unix_no_crt_riscv64.asm"
+			SYS_exit :: 93
 		}
 		@(link_name="_start_odin", linkage="strong", require)
 		_start_odin :: proc "c" (argc: i32, argv: [^]cstring) -> ! {

base/runtime/entry_unix_no_crt_riscv64.asm

@@ -0,0 +1,10 @@
+.text
+
+.globl _start
+
+_start:
+	ld a0, 0(sp)
+	addi a1, sp, 8
+	addi sp, sp, ~15
+	call _start_odin
+	ebreak

base/runtime/os_specific_linux.odin

@@ -12,6 +12,8 @@ _stderr_write :: proc "contextless" (data: []byte) -> (int, _OS_Errno) {
 		SYS_write :: uintptr(4)
 	} else when ODIN_ARCH == .arm32 {
 		SYS_write :: uintptr(4)
+	} else when ODIN_ARCH == .riscv64 {
+		SYS_write :: uintptr(64)
 	}
 
 	stderr :: 2