tools/nolibc: add a makefile to install headers

This provides a target "headers_standalone" which installs the nolibc's
arch-specific headers with "arch.h" taken from the current arch (or a
concatenation of both i386 and x86_64 for arch=x86), then installs
kernel headers. This creates a convenient sysroot which is directly
usable by a bare-metal compiler to create any executable.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/Makefile

+# SPDX-License-Identifier: GPL-2.0
+# Makefile for nolibc installation and tests
+include ../../scripts/Makefile.include
+
+# we're in ".../tools/include/nolibc"
+ifeq ($(srctree),)
+srctree := $(patsubst %/tools/include/,%,$(dir $(CURDIR)))
+endif
+
+nolibc_arch := $(patsubst arm64,aarch64,$(ARCH))
+arch_file := arch-$(nolibc_arch).h
+all_files := ctype.h errno.h nolibc.h signal.h std.h stdio.h stdlib.h string.h \
+             sys.h time.h types.h unistd.h
+
+# install all headers needed to support a bare-metal compiler
+all:
+
+# Note: when ARCH is "x86" we concatenate both x86_64 and i386
+headers:
+	$(Q)mkdir -p $(OUTPUT)sysroot
+	$(Q)mkdir -p $(OUTPUT)sysroot/include
+	$(Q)cp $(all_files) $(OUTPUT)sysroot/include/
+	$(Q)if [ "$(ARCH)" = "x86" ]; then      \
+		sed -e                          \
+		  's,^#ifndef _NOLIBC_ARCH_X86_64_H,#if !defined(_NOLIBC_ARCH_X86_64_H) \&\& defined(__x86_64__),' \
+		  arch-x86_64.h;                \
+		sed -e                          \
+		  's,^#ifndef _NOLIBC_ARCH_I386_H,#if !defined(_NOLIBC_ARCH_I386_H) \&\& !defined(__x86_64__),' \
+		  arch-i386.h;                  \
+	elif [ -e "$(arch_file)" ]; then        \
+		cat $(arch_file);               \
+	else                                    \
+		echo "Fatal: architecture $(ARCH) not yet supported by nolibc." >&2; \
+		exit 1;                         \
+	fi > $(OUTPUT)sysroot/include/arch.h
+
+headers_standalone: headers
+	$(Q)$(MAKE) -C $(srctree) headers
+	$(Q)$(MAKE) -C $(srctree) headers_install INSTALL_HDR_PATH=$(OUTPUT)/sysroot
+
+clean:
+	$(call QUIET_CLEAN, nolibc) rm -rf "$(OUTPUT)sysroot"