Build failure when using XCC toolchain

[sathishkuttan]

Describe the bug
With commit 9c2ac77, XCC is invoked with -nostdinc flag and this causes compilation errors. The -nostdinc flag seems to make XCC not look for header files in XtensaTools/xtensa-elf/include

To Reproduce
Steps to reproduce the behavior:

1. cd $ZEPHYR_BASE/tests/boards/intel_s1000_crb
2. mkdir build; cd build
3. cmake -DBOARD=intel_s1000_crb ..
4. make
5. See error

Expected behavior
Build successful

Impact
Can no longer build using XCC

Screenshots or console output

../../../../../include/toolchain/xcc.h:21:32: error: xtensa/config/core.h: No such file or directory
../../../../../arch/xtensa/include/xtensa_context.h:26:34: error: xtensa/config/tie.h: No such file or directory
../../../../../arch/xtensa/include/xtensa_context.h:27:32: error: xtensa/corebits.h: No such file or directory
../../../../../arch/xtensa/include/xtensa_context.h:28:37: error: xtensa/config/system.h: No such file or directory
../../../../../arch/xtensa/include/xtensa_context.h:29:40: error: xtensa/xtruntime-frames.h: No such file or directory

Environment (please complete the following information):

• OS: Linux
• Toolchain: Xtensa
• Commit SHA or Version used: 9c2ac77

Additional context
Temporarily removing -nostdinc flag for experimentation made the build successful.

[nashif]

this should fix it: #13675