eclass/sysroot: Add from Gentoo

It's from Gentoo commit fc1a2004794575b7935e8716501ec9f1d63ed95e.

sdk_container/src/third_party/portage-stable/eclass/sysroot.eclass

@@ -0,0 +1,85 @@
+# Copyright 2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+# @ECLASS: sysroot.eclass
+# @MAINTAINER:
+# cross@gentoo.org
+# @AUTHOR:
+# James Le Cuirot <chewi@gentoo.org>
+# @SUPPORTED_EAPIS: 7 8
+# @BLURB: Common functions for using a different sysroot (e.g. cross-compiling)
+
+case ${EAPI} in
+	7|8) ;;
+	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
+esac
+
+# @FUNCTION: qemu_arch
+# @DESCRIPTION:
+# Return the QEMU architecture name for the given target or CHOST. This name is
+# used in qemu-user binary filenames, e.g. qemu-ppc64le.
+qemu_arch() {
+	local target=${1:-${CHOST}}
+	case ${target} in
+		armeb*) echo armeb ;;
+		arm*) echo arm ;;
+		hppa*) echo hppa ;;
+		i?86*) echo i386 ;;
+		m68*) echo m68k ;;
+		mips64el*-gnuabi64) echo mips64el ;;
+		mips64el*-gnuabin32) echo mipsn32el ;;
+		mips64*-gnuabi64) echo mips64 ;;
+		mips64*-gnuabin32) echo mipsn32 ;;
+		powerpc64le*) echo ppc64le ;;
+		powerpc64*) echo ppc64 ;;
+		powerpc*) echo ppc ;;
+		*) echo "${target%%-*}" ;;
+	esac
+}
+
+# @FUNCTION: sysroot_make_run_prefixed
+# @DESCRIPTION:
+# Create a wrapper script for directly running executables within a sysroot
+# without changing the root directory. The path to that script is returned. If
+# no sysroot has been set, then this function returns unsuccessfully.
+#
+# The script explicitly uses QEMU if this is necessary and it is available in
+# this environment. It may otherwise implicitly use a QEMU outside this
+# environment if binfmt_misc has been used with the F flag. It is not feasible
+# to add a conditional dependency on QEMU.
+sysroot_make_run_prefixed() {
+	[[ -z ${SYSROOT} ]] && return 1
+
+	local SCRIPT="${T}"/sysroot-run-prefixed
+	local QEMU_ARCH=$(qemu_arch)
+
+	# Both methods below might need help to find GCC's libraries. GCC might not
+	# even be installed in the SYSROOT. Note that Clang supports this flag too.
+	local LIBGCC=$($(tc-getCC) ${CPPFLAGS} ${CFLAGS} ${LDFLAGS} -print-libgcc-file-name)
+
+	if [[ ${QEMU_ARCH} == $(qemu_arch "${CBUILD}") ]]; then
+		# glibc: ld.so is a symlink, ldd is a binary.
+		# musl: ld.so doesn't exist, ldd is a symlink.
+		local DLINKER=$(find "${ESYSROOT}"/usr/bin/{ld.so,ldd} -type l -print -quit 2>/dev/null || die "failed to find dynamic linker")
+
+		# musl symlinks ldd to ld-musl.so to libc.so. We want the ld-musl.so
+		# path, not the libc.so path, so don't resolve the symlinks entirely.
+		DLINKER=$(readlink -ev "${DLINKER}" || die "failed to find dynamic linker")
+
+		# Using LD_LIBRARY_PATH to set the prefix is not perfect, as it doesn't
+		# adjust RUNPATHs, but it is probably good enough.
+		install -m0755 /dev/stdin "${SCRIPT}" <<-EOF || die
+			#!/bin/sh
+			LD_LIBRARY_PATH="\${LD_LIBRARY_PATH}\${LD_LIBRARY_PATH+:}${LIBGCC%/*}:${ESYSROOT}/$(get_libdir):${ESYSROOT}/usr/$(get_libdir)" exec "${DLINKER}" "\${@}"
+		EOF
+	else
+		# Use QEMU's environment variables rather than its command line
+		# arguments to cover both explicit and implicit QEMU usage.
+		install -m0755 /dev/stdin "${SCRIPT}" <<-EOF || die
+			#!/bin/sh
+			QEMU_SET_ENV="\${QEMU_SET_ENV}\${QEMU_SET_ENV+,}LD_LIBRARY_PATH=\${LD_LIBRARY_PATH}\${LD_LIBRARY_PATH+:}${LIBGCC%/*}" QEMU_LD_PREFIX="${SYSROOT}" exec $(type -P "qemu-${QEMU_ARCH}") "\${@}"
+		EOF
+	fi
+
+	echo "${SCRIPT}"
+}