Respect JULIA_CPU_TARGET set in Make.inc when building pkgimgs (#51886)

Co-authored-by: Elliot Saba <staticfloat@gmail.com>

pkgimage.mk

@@ -29,6 +29,10 @@ all-debug:   $(addprefix cache-debug-, $(STDLIBS))
 
 define stdlib_builder
 ifneq ($(filter $(1),$(INDEPENDENT_STDLIBS)),)
+# Define target-specific export for `JULIA_CPU_TARGET`
+$$(BUILDDIR)/stdlib/$1.release.image: export JULIA_CPU_TARGET=$(JULIA_CPU_TARGET)
+$$(BUILDDIR)/stdlib/$1.debug.image: export JULIA_CPU_TARGET=$(JULIA_CPU_TARGET)
+
 $$(BUILDDIR)/stdlib/$1.release.image: $$($1_SRCS) $$(addsuffix .release.image,$$(addprefix $$(BUILDDIR)/stdlib/,$2)) $(build_private_libdir)/sys.$(SHLIB_EXT)
 	@$$(call PRINT_JULIA, $$(call spawn,$$(JULIA_EXECUTABLE)) --startup-file=no --check-bounds=yes -e 'Base.compilecache(Base.identify_package("$1"))')
 	@$$(call PRINT_JULIA, $$(call spawn,$$(JULIA_EXECUTABLE)) --startup-file=no --pkgimage=no -e 'Base.compilecache(Base.identify_package("$1"))')