Rename "--with-base-isa" to "--with-arch"

This matches the GCC configure option.

Makefile.in

@@ -16,7 +16,7 @@ glibc_url := $(GNU_MIRROR)/glibc/glibc-$(glibc_version).tar.gz
 newlib_url := ftp://sourceware.org/pub/newlib/newlib-$(newlib_version).tar.gz
 
 XLEN ?= @XLEN@
-BASE_ISA ?= @BASE_ISA@
+ARCH ?= @ARCH@
 SYSROOT := $(INSTALL_DIR)/sysroot
 
 SHELL := /bin/sh
@@ -161,7 +161,7 @@ stamps/build-gcc-linux-stage1: src/gcc stamps/build-binutils-linux \
 		--disable-nls \
 		$(MULTILIB_FLAGS) \
 		--disable-bootstrap \
-		--with-arch=RV$(XLEN)$(BASE_ISA)
+		--with-arch=RV$(XLEN)$(ARCH)
 	$(MAKE) -C $(notdir $@) inhibit-libc=true all-gcc
 	$(MAKE) -C $(notdir $@) inhibit-libc=true install-gcc
 	$(MAKE) -C $(notdir $@) inhibit-libc=true all-target-libgcc
@@ -185,7 +185,7 @@ stamps/build-gcc-linux-stage2: src/gcc stamps/build-glibc-linux$(XLEN) \
 		--disable-nls \
 		$(MULTILIB_FLAGS) \
 		--disable-bootstrap \
-		--with-arch=RV$(XLEN)$(BASE_ISA)
+		--with-arch=RV$(XLEN)$(ARCH)
 	$(MAKE) -C $(notdir $@)
 	$(MAKE) -C $(notdir $@) install
 	mkdir -p $(dir $@) && touch $@
@@ -226,7 +226,7 @@ stamps/build-gcc-newlib: src/newlib-gcc stamps/build-binutils-newlib
 		--disable-libquadmath \
 		--disable-libgomp \
 		--disable-nls \
-		--with-arch=RV$(XLEN)$(BASE_ISA)
+		--with-arch=RV$(XLEN)$(ARCH)
 	$(MAKE) -C $(notdir $@) inhibit-libc=true
 	$(MAKE) -C $(notdir $@) install
 	mkdir -p $(dir $@) && touch $@

configure.ac

@@ -56,11 +56,11 @@ AC_ARG_WITH(xlen,
 	AC_SUBST(XLEN, 64)
 	)
 
-AC_ARG_WITH(base-isa,
-	[AS_HELP_STRING([--with-base-isa=G/I],
-		[Sets the base RISC-V ISA to either G or I, defaults to G])],
-	AC_SUBST(BASE_ISA, $withval),
-	AC_SUBST(BASE_ISA, G)
+AC_ARG_WITH(arch,
+	[AS_HELP_STRING([--with-arch=G],
+		[Sets the base RISC-V ISA, defaults to G])],
+	AC_SUBST(ARCH, $withval),
+	AC_SUBST(ARCH, G)
 	)
 
 AC_ARG_ENABLE(atomic,