tools/nolibc: add new crt.h with _start_c

As the environ and _auxv support added for nolibc, the assembly _start
function becomes more and more complex and therefore makes the porting
of nolibc to new architectures harder and harder.

To simplify portability, this C version of _start_c() is added to do
most of the assembly start operations in C, which reduces the complexity
a lot and will eventually simplify the porting of nolibc to the new
architectures.

The new _start_c() only requires a stack pointer argument, it will find
argc, argv, envp/environ and _auxv for us, and then call main(),
finally, it exit() with main's return status. With this new _start_c(),
the future new architectures only require to add very few assembly
instructions.

As suggested by Thomas, users may use a different signature of main
(e.g. void main(void)), a _nolibc_main alias is added for main to
silence the warning about potential conflicting types.

As suggested by Willy, the code is carefully polished for both smaller
size and better readability with local variables and the right types.
Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20230715095729.GC24086@1wt.eu/Suggested-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/90fdd255-32f4-4caf-90ff-06456b53dac3@t-8ch.de/Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

tools/include/nolibc/Makefile

@@ -27,6 +27,7 @@ nolibc_arch := $(patsubst arm64,aarch64,$(ARCH))
 arch_file := arch-$(nolibc_arch).h
 all_files := \
 		compiler.h \
+		crt.h \
 		ctype.h \
 		errno.h \
 		nolibc.h \

tools/include/nolibc/crt.h

+/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
+/*
+ * C Run Time support for NOLIBC
+ * Copyright (C) 2023 Zhangjin Wu <falcon@tinylab.org>
+ */
+
+#ifndef _NOLIBC_CRT_H
+#define _NOLIBC_CRT_H
+
+char **environ __attribute__((weak));
+const unsigned long *_auxv __attribute__((weak));
+
+static void exit(int);
+
+void _start_c(long *sp)
+{
+	long argc;
+	char **argv;
+	char **envp;
+	const unsigned long *auxv;
+	/* silence potential warning: conflicting types for 'main' */
+	int _nolibc_main(int, char **, char **) __asm__ ("main");
+
+	/*
+	 * sp  :    argc          <-- argument count, required by main()
+	 * argv:    argv[0]       <-- argument vector, required by main()
+	 *          argv[1]
+	 *          ...
+	 *          argv[argc-1]
+	 *          null
+	 * environ: environ[0]    <-- environment variables, required by main() and getenv()
+	 *          environ[1]
+	 *          ...
+	 *          null
+	 * _auxv:   _auxv[0]      <-- auxiliary vector, required by getauxval()
+	 *          _auxv[1]
+	 *          ...
+	 *          null
+	 */
+
+	/* assign argc and argv */
+	argc = *sp;
+	argv = (void *)(sp + 1);
+
+	/* find environ */
+	environ = envp = argv + argc + 1;
+
+	/* find _auxv */
+	for (auxv = (void *)envp; *auxv++;)
+		;
+	_auxv = auxv;
+
+	/* go to application */
+	exit(_nolibc_main(argc, argv, envp));
+}
+
+#endif /* _NOLIBC_CRT_H */