from both the acpi module build directory and a kernel build directory.
The latter didn't work when one attempted to build a kernel which had
"device acpi" with the "make kernel-toolchain buildkernel" command
because a cross-compiler couldn't find anything in the standard system
include path (it's empty in the kernel-toolchain case).
Fix this by passing a better root path to kernel headers (src/sys)
which works for both cases, kernel and module (-I@ only worked for
module).
Also, while here, pass -nostdinc (and a different spelling for icc) --
it's a feature that the kernel source tree is self-contained, and this
change enforces this.
Reported by: glebius
4962065404