Add hints on CARCH/CBUILD for x86_64 and powerpc.

abuild.conf

 CARCH=x86
 CBUILD=i486-alpine-linux-uclibc
+
+# for x86_64 we want:
+# CARCH=x86_64
+# CBUILD=x86_64-alpine-linux-uclibc
+
+# for powerpc we want:
+# CARCH=powerpc
+# CBUILD=powerpc-alpine-linux-uclibc
+
 CHOST=$CBUILD
 CTARGET=$CHOST