Pull Kbuild updates from Masahiro Yamada:
 "Kbuild core:
   - remove unneeded $(call cc-option,...) switches
   - consolidate Clang compiler flags into CLANG_FLAGS
   - announce the deprecation of SUBDIRS
   - fix single target build for external module
   - simplify the dependencies of 'prepare' stage targets
   - allow fixdep to directly write to .*.cmd files
   - simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS
   - change if_changed_rule to accept multi-line recipe
   - move .SECONDARY special target to scripts/Kbuild.include
   - remove redundant 'set -e'
   - improve parallel execution for CONFIG_HEADERS_CHECK
   - misc cleanups

  Treewide fixes and cleanups
   - set Clang flags correctly for PowerPC boot images
   - fix UML build error with CONFIG_GCC_PLUGINS
   - remove unneeded patterns from .gitignore files
   - refactor firmware/Makefile
   - remove unneeded rules for *offsets.s
   - avo...