[clang][driver] allow `-arch arm64` to be used to build for mac when on Apple Silicon Mac without explicit `-target`

[arphaman]

Summary

This patch allows a user to compile for the arm64-apple-macos target when invoking a clang running on an Apple Silicon machine by passing -arch arm64 only.

[steven_wu]

LGTM.

Not sure if it makes more sense to break the patch into two commits:

• config.guess change is for building the correct host triple on apple silicon machine without explicitly specify it.
• the driver change is for better default on Apple silicon Mac.

[arphaman]

In D82428#2110506, @steven_wu wrote:

LGTM.

Not sure if it makes more sense to break the patch into two commits:

• config.guess change is for building the correct host triple on apple silicon machine without explicitly specify it.
• the driver change is for better default on Apple silicon Mac.

That's a good idea, I will do that.

llvm/cmake/config.guess

case $UNAME_PROCESSOR in
     if [ "$CC_FOR_BUILD" != 'no_compiler_found' ]; then
       if (echo '#ifdef __LP64__'; echo IS_64BIT_ARCH; echo '#endif') | \
           (CCOPTS= $CC_FOR_BUILD -E - 2>/dev/null) | \
           grep IS_64BIT_ARCH >/dev/null
       then
           UNAME_PROCESSOR="x86_64"
       fi
     fi ;;
+    arm)
+    eval $set_cc_for_build
+    if [ "$CC_FOR_BUILD" != 'no_compiler_found' ]; then
+      if (echo '#ifdef __LP64__'; echo IS_64BIT_ARCH; echo '#endif') | \
+        (CCOPTS= $CC_FOR_BUILD -E - 2>/dev/null) | \
+        grep IS_64BIT_ARCH >/dev/null
+      then
+        if (echo '#ifdef __PTRAUTH_INTRINSICS__'; echo HAS_AUTH; echo '#endif') | \
+          (CCOPTS= $CC_FOR_BUILD -E - 2>/dev/null) | \
+          grep HAS_AUTH >/dev/null
+        then
+          UNAME_PROCESSOR="arm64e"
+        else
+          UNAME_PROCESSOR="arm64"
+        fi
+      fi
+    fi ;;
       unknown) UNAME_PROCESSOR=powerpc ;;
   esac
   echo ${UNAME_PROCESSOR}-apple-darwin${UNAME_RELEASE}
   exit ;;
     *:procnto*:*:* | *:QNX:[0123456789]*:*)
   UNAME_PROCESSOR=`uname -p`
   if test "$UNAME_PROCESSOR" = "x86"; then
     UNAME_PROCESSOR=i386