Fix OS_ARCH detection for IRIX64.

Thanks to johnv@adacel.com.au (John Mark Vandenberg) for the patch.
Bug #86346 r=cls

mstone/config/config.mk

@@ -68,7 +68,7 @@ ifeq ($(OS_ARCH), WINNT)
 	PERL_LIB_OS_FILES = $(PERL5_IMPORT)/lib/$(PERL_OS)/*.pm
 endif
 ifeq ($(OS_ARCH), IRIX64)
-	ARCH	= IRIX
+	OS_ARCH	= IRIX
 endif
 ifeq ($(OS_ARCH), IRIX)
 	# MIPSpro Compilers: Version 7.2.1
@@ -156,4 +156,4 @@ ifeq ($(BUILD_VARIANT),release)
 STRIP := strip
 else
 STRIP := true
-endif
+endif