x/sys/unix: use xcrun to find OS X and iOS SDKs in mkall.sh.

This makes the script resistant to changes in the SDK location, which
apparently happens from time to time.

While I was at it, made another cosmetic touch-up: use a .gitignore file
to stop `git status` from showing the _obj/ directory generated by
mkall.sh.

Change-Id: Ia706114d94e324ab443f6f5d6aa7c0c9f8a84620
Reviewed-on: https://go-review.googlesource.com/14192Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/.gitignore

+_obj/

unix/mkall.sh

@@ -116,12 +116,12 @@ _* | *_ | _)
 darwin_386)
 	mkerrors="$mkerrors -m32"
 	mksyscall="./mksyscall.pl -l32"
-	mksysnum="./mksysnum_darwin.pl /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/sys/syscall.h"
+	mksysnum="./mksysnum_darwin.pl $(xcrun --show-sdk-path --sdk macosx)/usr/include/sys/syscall.h"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 darwin_amd64)
 	mkerrors="$mkerrors -m64"
-	mksysnum="./mksysnum_darwin.pl /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include/sys/syscall.h"
+	mksysnum="./mksysnum_darwin.pl $(xcrun --show-sdk-path --sdk macosx)/usr/include/sys/syscall.h"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 darwin_arm)
@@ -131,7 +131,7 @@ darwin_arm)
 	;;
 darwin_arm64)
 	mkerrors="$mkerrors -m64"
-	mksysnum="./mksysnum_darwin.pl /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/sys/syscall.h"
+	mksysnum="./mksysnum_darwin.pl $(xcrun --show-sdk-path --sdk iphoneos)/usr/include/sys/syscall.h"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 dragonfly_386)

unix/zsysnum_darwin_arm64.go

-// mksysnum_darwin.pl /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/sys/syscall.h
+// mksysnum_darwin.pl /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.4.sdk/usr/include/sys/syscall.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
 // +build arm64,darwin