multilib-build.eclass: Use split assignment/readonly calls

Use split assignments and readonly calls rather than combined form since
the latter works unreliably for reason unknown.

eclass/multilib-build.eclass

@@ -38,7 +38,7 @@ inherit multibuild multilib
 # Please contact multilib before modifying this list. This way we can
 # ensure that every *preliminary* work is done and the multilib can be
 # extended safely.
-readonly _MULTILIB_FLAGS=(
+_MULTILIB_FLAGS=(
 	abi_x86_32:x86,x86_fbsd,x86_freebsd,x86_linux,x86_macos,x86_solaris
 	abi_x86_64:amd64,amd64_fbsd,x64_freebsd,amd64_linux,x64_macos,x64_solaris
 	abi_x86_x32:x32
@@ -50,6 +50,7 @@ readonly _MULTILIB_FLAGS=(
 	abi_s390_32:s390
 	abi_s390_64:s390x
 )
+readonly _MULTILIB_FLAGS
 
 # @ECLASS-VARIABLE: MULTILIB_COMPAT
 # @DEFAULT_UNSET
@@ -124,7 +125,8 @@ _multilib_build_set_globals() {
 	local usedeps=${flags[@]/%/(-)?}
 
 	IUSE=${flags[*]}
-	readonly MULTILIB_USEDEP=${usedeps// /,}
+	MULTILIB_USEDEP=${usedeps// /,}
+	readonly MULTILIB_USEDEP
 }
 _multilib_build_set_globals
 unset -f _multilib_build_set_globals