Align Kconfig and Kbuild logic to Linux 4.20 release with minimal impact
on files outside of this scope.

Our previous Kconfig sync was done by commit 587e4a42 ("kconfig /
kbuild: Re-sync with Linux 4.19").

As part of this re-sync, a few related changes from previous Linux
releases were found to have been missed and merged in, and are not in
the following list.

The imported Linux commits are:

[From prior to v4.19]
b1e0d8b70fa3 kbuild: Fix gcc -x syntax
a4353898980c kconfig: add CC_IS_GCC and GCC_VERSION
469cb7376c06 kconfig: add CC_IS_CLANG and CLANG_VERSION

[From v4.19 to v4.20]
487c7c7702ab kbuild: prefix Makefile.dtbinst path with $(srctree) unconditionally
0d91bf584fe5 kbuild: remove old check for CFLAGS use
25815cf5ffec kbuild: hide most of targets when running config or mixed targets
00d78ab2ba75 kbuild: remove dead code in cmd_files calculation in top Makefile
23066c3f4e21 Compiler Attributes: enable -Wstringop-truncation on W=1 (gcc >= 8)
37c8a5faf...