xtensa xt-xcc inline warning

[hongshui3000]

Describe the bug
xtensa xcc inline warning

To Reproduce
Steps to reproduce the behavior:
west build -b qemu_xtensa samples/hello_world

Logs and console output
E:/zephyrproject/zephyr/kernel/sched.c:699: warning: C99 inline functions are not supported; using GNU89
E:/zephyrproject/zephyr/kernel/sched.c:699: warning: to disable this warning use -fgnu89-inline or the gnu_inline function attribute
../kernel/include/ksched.h:58: warning: 'z_find_first_thread_to_unpend' declared inline after being called
../kernel/include/ksched.h:58: warning: previous declaration of 'z_find_first_thread_to_unpend' was here
E:/work/rtos_work/zephyrproject/zephyr/kernel/sched.c:719: warning: C99 inline functions are not supported; using GNU89
../kernel/include/ksched.h:43: warning: 'z_unpend_thread_no_timeout' declared inline after being called
../kernel/include/ksched.h:43: warning: previous declaration of 'z_unpend_thread_no_timeout' was here
E:/zephyrproject/zephyr/kernel/sched.c:964: warning: C99 inline functions are not supported; using GNU89
E:/zephyrproject/zephyr/kernel/sched.c:1080: warning: C99 inline functions are not supported; using GNU89
E:/zephyrproject/zephyr/kernel/sched.c:1088: warning: C99 inline functions are not supported; using GNU89
[65/65] Linking C executable zephyr\zephyr.elf

Environment (please complete the following information):

• OS: ( Windows)
• Toolchain (xcc)

[dcpleung]

This is due to XCC not supporting C99 inline as XCC is based on GCC 4.2. To get rid of these warning requires refactoring lots of scheduler code, which is undesirable to simply support an outdated compiler. My recommendation is to try a newer XCC release which has Clang support (e.g. has xt-clang executable), as it should have C99 inlining support alreay.

Closing this as "won't fix".