Kbuild fixes for v4.13

- fix linker script regression caused by dead code elimination support
 
 - fix typos and outdated comments
 
 - specify kselftest-clean as a PHONY target
 
 - fix "make dtbs_install" when $(srctree) includes shell special
   characters like '~'
 
 - Move -fshort-wchar to the global option list because defining it
   partially emits warnings
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Merge tag 'kbuild-fixes-v4.13' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild

Pull Kbuild fixes from Masahiro Yamada:

 - fix linker script regression caused by dead code elimination support

 - fix typos and outdated comments

 - specify kselftest-clean as a PHONY target

 - fix "make dtbs_install" when $(srctree) includes shell special
   characters like '~'

 - Move -fshort-wchar to the global option list because defining it
   partially emits warnings

* tag 'kbuild-fixes-v4.13' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild:
  kbuild: update comments of Makefile.asm-generic
  kbuild: Do not use hyphen in exported variable name
  Makefile: add kselftest-clean to PHONY target list
  Kbuild: use -fshort-wchar globally
  fixdep: trivial: typo fix and correction
  kbuild: trivial cleanups on the comments
  kbuild: linker script do not match C names unless LD_DEAD_CODE_DATA_ELIMINATION is configured

drivers/xen/Makefile

@@ -7,9 +7,6 @@ obj-y	+= xenbus/
 nostackp := $(call cc-option, -fno-stack-protector)
 CFLAGS_features.o			:= $(nostackp)
 
-CFLAGS_efi.o				+= -fshort-wchar
-LDFLAGS					+= $(call ld-option, --no-wchar-size-warning)
-
 dom0-$(CONFIG_ARM64) += arm-device.o
 dom0-$(CONFIG_PCI) += pci.o
 dom0-$(CONFIG_USB_SUPPORT) += dbgp.o