Export ARCH64 to config/Makefile as well as m.h

Allows it to be used more easily in the build system.

Makefile

@@ -1203,7 +1203,7 @@ check_arch:
 
 .PHONY: check_all_arches
 check_all_arches:
-ifneq ($(shell grep -E '^\#define ARCH_SIXTYFOUR$$' byterun/caml/m.h 2> /dev/null),)
+ifeq ($(ARCH64),true)
 	@STATUS=0; \
 	 for i in $(ARCHES); do \
 	   $(MAKE) --no-print-directory check_arch ARCH=$$i || STATUS=1; \

config/Makefile.mingw

@@ -178,6 +178,7 @@ ARCMD=$(TOOLPREF)ar
 
 ### Name of architecture for the native-code compiler
 ARCH=i386
+ARCH64=false
 
 ### Name of architecture model for the native-code compiler.
 MODEL=default

config/Makefile.mingw64

@@ -178,6 +178,7 @@ ARCMD=$(TOOLPREF)ar
 
 ### Name of architecture for the native-code compiler
 ARCH=amd64
+ARCH64=true
 
 ### Name of architecture model for the native-code compiler.
 MODEL=default

config/Makefile.msvc

@@ -173,6 +173,7 @@ ARCMD=
 
 ### Name of architecture for the native-code compiler
 ARCH=i386
+ARCH64=false
 
 ### Name of architecture model for the native-code compiler.
 MODEL=default

config/Makefile.msvc64

@@ -176,6 +176,7 @@ ARCMD=
 
 ### Name of architecture for the native-code compiler
 ARCH=amd64
+ARCH64=true
 
 ### Name of architecture model for the native-code compiler.
 MODEL=default

configure

@@ -670,9 +670,11 @@ if test "$?" -eq 0; then
   case "$3" in
     4) inf "OK, this is a regular 32 bit architecture."
        echo "#undef ARCH_SIXTYFOUR" >> m.h
+       config ARCH64 "false"
        arch64=false;;
     8) inf "Wow! A 64 bit architecture!"
        echo "#define ARCH_SIXTYFOUR" >> m.h
+       config ARCH64 "true"
        arch64=true;;
     *) err "This architecture seems to be neither 32 bits nor 64 bits.\n" \
            "OCaml won't run on this architecture.";;