multilib.eclass: derive BUILD_* before appending ABI flags

Per [1], we cannot have ${MULTILIB_USEDEP} in BDEPEND because it would
break cross-compiles, as ${MULTILIB_USEDEP} specifies ABIs of CHOST, not
of CBUILD.

However, this leads to a problem, as multilib.eclass does not preserve
the default build-machine toolchain before it appends the ABI flags to
the host-machine toolchain. Thus, when $(tc-getBUILD_CC) later pulls a
default value for BUILD_CC from CC, the ABI flags have already been
appended to CC, and BUILD_CC inherits those flags, thus causing any
build-time helper executables to be built for a non-default ABI, whose
build-time dependencies may not be installed in ${BROOT} since
${MULTILIB_USEDEP} makes no sense in BDEPEND.

Note, the behavior is inconsistent if the user has set BUILD_CC and
friends via their package.env, as then the ABI flags are *not* applied
to the build-machine toolchain via defaulting from the host-machine
toolchain.

This commit fixes up multilib.eclass so that it derives the build-
machine toolchain variables _before_ it appends the ABI flags to the
host-machine toolchain variables and so that it overrides CBUILD to the
default ABI in the non-cross-compiling case. The result is that build-
time helper executables are always built for the build machine's
_default_ ABI, even in the non-cross-compiling case.

[1] https://bugs.gentoo.org/960506#c9

Bug: https://bugs.gentoo.org/960506
Signed-off-by: Matt Whitlock <gentoo@mattwhitlock.name>
Signed-off-by: James Le Cuirot <chewi@gentoo.org>

eclass/multilib.eclass

@@ -487,6 +487,17 @@ multilib_toolchain_setup() {
 	local save_restore_variables=(
 		CBUILD
 		CHOST
+		BUILD_AR
+		BUILD_CC
+		BUILD_CXX
+		BUILD_LD
+		BUILD_NM
+		BUILD_OBJCOPY
+		BUILD_PKG_CONFIG
+		BUILD_RANLIB
+		BUILD_READELF
+		BUILD_STRINGS
+		BUILD_STRIP
 		AR
 		CC
 		CXX
@@ -525,17 +536,31 @@ multilib_toolchain_setup() {
 		done
 		export _DEFAULT_ABI_SAVED="true"
 
-		# Set CBUILD only if not cross-compiling.
-		if [[ ${CBUILD} == "${CHOST}" ]]; then
-			export CBUILD=$(get_abi_CHOST $1)
-		fi
-
 		# Set the CHOST native first so that we pick up the native
 		# toolchain and not a cross-compiler by accident #202811.
 		#
 		# Make sure ${save_restore_variables[@]} list matches below.
 		export CHOST=$(get_abi_CHOST ${DEFAULT_ABI})
 
+		# Set CBUILD only if not cross-compiling.
+		if [[ "${_abi_saved_CBUILD:-${_abi_saved_CHOST}}" == "${_abi_saved_CHOST}" ]]; then
+			export CBUILD=${CHOST}
+		fi
+
+		# Derive the build-machine toolchain variables before we
+		# override the host-machine toolchain variables.
+		export BUILD_AR="$(tc-getBUILD_AR)" # Avoid 'ar', use "${CBUILD}-ar"
+		export BUILD_CC="$(tc-getBUILD_CC)" # Default ABI
+		export BUILD_CXX="$(tc-getBUILD_CXX)" # Default ABI
+		export BUILD_LD="$(tc-getBUILD_LD)" # Default ABI
+		export BUILD_NM="$(tc-getBUILD_NM)" # Avoid 'nm', use "${CBUILD}-nm"
+		export BUILD_OBJCOPY="$(tc-getBUILD_OBJCOPY)" # Avoid 'objcopy', use "${CBUILD}-objcopy"
+		export BUILD_PKG_CONFIG="$(tc-getBUILD_PKG_CONFIG)"
+		export BUILD_RANLIB="$(tc-getBUILD_RANLIB)" # Avoid 'ranlib', use "${CBUILD}-ranlib"
+		export BUILD_READELF="$(tc-getBUILD_READELF)" # Avoid 'readelf', use "${CBUILD}-readelf"
+		export BUILD_STRINGS="$(tc-getBUILD_STRINGS)" # Avoid 'strings', use "${CBUILD}-strings"
+		export BUILD_STRIP="$(tc-getBUILD_STRIP)" # Avoid 'strip', use "${CBUILD}-strip"
+
 		export AR="$(tc-getAR)" # Avoid 'ar', use '${CHOST}-ar'
 		export CC="$(tc-getCC) $(get_abi_CFLAGS)"
 		export CXX="$(tc-getCXX) $(get_abi_CFLAGS)"
@@ -556,6 +581,12 @@ multilib_toolchain_setup() {
 		export PKG_CONFIG_PATH=${ESYSROOT}/usr/share/pkgconfig
 		export PKG_CONFIG_SYSTEM_INCLUDE_PATH=${ESYSROOT}/usr/include
 		export PKG_CONFIG_SYSTEM_LIBRARY_PATH=${ESYSROOT}/$(get_libdir):${ESYSROOT}/usr/$(get_libdir)
+
+		# Also set CBUILD so as not to trigger cross-compilation
+		# mode falsely in build systems.
+		if [[ "${_abi_saved_CBUILD:-${_abi_saved_CHOST}}" == "${_abi_saved_CHOST}" ]]; then
+			export CBUILD=${CHOST}
+		fi
 	fi
 }