Update config scripts to compile for Apple Silicon (arm64)

Detect Apple Silicon (arm64) during configuration for compilation
on macOS 11.0 (beta at this time).

.gitignore

@@ -38,6 +38,7 @@ armv7l-unknown-linux-gnueabihf
 i686-pc-linux-gnu
 x86_64-unknown-linux-gnu
 i386-apple-darwin[0-9]*.[0-9]*.[0-9]*
+arm-apple-darwin[0-9]*.[0-9]*.[0-9]*
 x86_64-apple-darwin[0-9]*.[0-9]*.[0-9]*
 x86_64-unknown-freebsd[0-9]*.[0-9]*
 sparc-sun-solaris[0-9]*.[0-9]*

erts/aclocal.m4

@@ -2071,6 +2071,9 @@ case "$THR_LIB_NAME" in
 	      macppc | ppc | powerpc | "Power Macintosh")
 	      	    ethr_native_atomic_implementation=ethread
 	    	    ethr_have_native_atomics=yes;;
+	      arm64)
+	      	    ethr_native_atomic_implementation=ethread
+	    	    ethr_have_native_atomics=yes;;
 	      tile)
 	            ethr_native_atomic_implementation=ethread
 	    	    ethr_have_native_atomics=yes;;
@@ -2822,6 +2825,7 @@ AC_DEFUN([LM_HARDWARE_ARCH], [
     ppc64)	ARCH=ppc64;;
     ppc64le)	ARCH=ppc64le;;
     "Power Macintosh")	ARCH=ppc;;
+    arm64)	ARCH=arm64;;
     armv5b)	ARCH=arm;;
     armv5teb)	ARCH=arm;;
     armv5tel)	ARCH=arm;;

erts/configure.in

@@ -750,7 +750,7 @@ dnl First remove common_tests skip file.
 
 dnl Adjust LDFLAGS to allow 64bit linkage on DARWIN
 case $ARCH-$OPSYS in
-	amd64-darwin*)
+	amd64-darwin*|arm64-darwin*)
 		AC_MSG_NOTICE([Adjusting LDFLAGS to cope with 64bit Darwin])
 		case $LDFLAGS in
 			*-m64*)

make/configure.in

@@ -396,7 +396,7 @@ if test $CROSS_COMPILING = no; then
 	   }
 	   [case "$macosx_version" in
 	       [1-9][0-9].[0-9])
-	          int_macosx_version=`echo $macosx_version | sed 's|\([^\.]*\)\.\([^\.]*\)|\1\2|'`;;
+	          int_macosx_version=`echo $macosx_version | sed 's|\([^\.]*\)\.\([^\.]*\)|\10\200|'`;;
 	       [1-9][0-9].[0-9].[0-9])
 	          int_macosx_version=`echo $macosx_version | sed 's|\([^\.]*\)\.\([^\.]*\)\.\([^\.]*\)|\1\2\3|'`;;
 	       [1-9][0-9].[1-9][0-9])