arch/x86/boot: Use prefix map to avoid embedded paths

It was observed that the kernel embeds the absolute build path in the
x86 boot image when the __FILE__ macro is expanded.

> From https://bugzilla.yoctoproject.org/show_bug.cgi?id=13458:

  If you turn on the buildpaths QA test, or try a reproducible build, you
  discover that the kernel image contains build paths.

  $ strings bzImage-5.0.19-yocto-standard |grep tmp/
  out of pgt_buf in
  /data/poky-tmp/reproducible/tmp/work-shared/qemux86-64/kernel-source/arch/x86/boot/compressed/kaslr_64.c!?

  But what's this in the top-level Makefile:

  $ git grep prefix-map
  Makefile:KBUILD_CFLAGS  += $(call
  cc-option,-fmacro-prefix-map=$(srctree)/=)

  So the __FILE__ shouldn't be using the full path.  However
  arch/x86/boot/compressed/Makefile has this:

  KBUILD_CFLAGS := -m$(BITS) -O2

  So that clears KBUILD_FLAGS, removing the -fmacro-prefix-map option.

Use -fmacro-prefix-map to have relative paths in the boot image too.

 [ bp: Massage commit message and put the KBUILD_CFLAGS addition in
   ..boot/Makefile after the KBUILD_AFLAGS assignment because gas
   doesn't support -fmacro-prefix-map. ]

Signed-off-by: Bruce Ashfield <[email protected]>
Signed-off-by: Ross Burton <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: George Rimar <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=204333

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