Pass the correct build flags to jemalloc

Apparently the standard --build and --host flags don't actually
_do_ anything. This re-uses the libuv flags, since they are the
same for getting jemalloc to cross-compile

mk/rt.mk

@@ -212,9 +212,15 @@ endif
 
 $$(JEMALLOC_LIB_$(1)_$(2)):
 	cd $$(CFG_BUILD_DIR)/rt/$(1)/stage$(2)/jemalloc; $(S)src/rt/jemalloc/configure \
-		--disable-experimental --build=$(CFG_BUILD_TRIPLE) --host=$(1)
+		--disable-experimental --build=$(CFG_BUILD_TRIPLE) --host=$(1) \
+		EXTRA_CFLAGS="$$(CFG_GCCISH_CFLAGS) $$(LIBUV_FLAGS_$$(HOST_$(1))) $$(SNAP_DEFINES)" \
+		LDFLAGS="$$(CFG_GCCISH_LINK_FLAGS) $$(LIBUV_FLAGS_$$(HOST_$(1)))" \
+		CC="$$(CC_$(1))" \
+		CXX="$$(CXX_$(1))" \
+		AR="$$(AR_$(1))"
 	$$(Q)$$(MAKE) -C $$(CFG_BUILD_DIR)/rt/$(1)/stage$(2)/jemalloc
 
+
 # These could go in rt.mk or rustllvm.mk, they're needed for both.
 
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