Modify build tool to support ARCH=aarch64 (#225)

sys/src/9/aarch64/BUILDME

@@ -1,5 +1,5 @@
 ARCH=aarch64  \
-STRIP=aarch64-none-eabi-strip \
-CC=aarch64-none-eabi-gcc \
-LD=aarch64-none-eabi-ld \
+STRIP=aarch64-none-elf-strip \
+CC=aarch64-none-elf-gcc \
+LD=aarch64-none-elf-ld \
 ../../../../util/build cpu.json

util/src/harvey/cmd/build/doc.go

@@ -161,7 +161,7 @@ See the build godoc for more information about the buildfile format.
 
 ENVIRONMENT
 
-ARCH is needed. Current acceptable vaules are: amd64
+ARCH is needed. Current acceptable vaules are: aarch64 amd64 riscv
 
 HARVEY may be supplied to point at a harvey tree. The default on harvey is "/".
 The default on Linux and OSX is to attempt to find the top level of a git

util/src/harvey/cmd/mksys/mksys.go

@@ -110,6 +110,8 @@ func main() {
 	case "sys_harvey.s":
 		a := os.Getenv("ARCH")
 		switch a {
+		case "aarch64":
+			usage("aarch64 support is incomplete")
 		case "amd64":
 		case "riscv":
 			usage("riscv support is incomplete")