toolchain.h: fix __in_section_unique()
toolchain.h: fix __in_section_unique()

_FILE_PATH_HASH appears to be a legacy Diab-ism that doesn't
expand to anything in GCC.

As a result, when linking the combined binary, it's quite
possible that objects in separate C files would be merged
instead of truly being in their own section. This can confound
--gc-sections and result in unused objects still being in
the final binary if one of the other objects with the same
generated section name was actually used.

We instead just use __FILE__. This results in sometimes absurdly-
long section names in the intermediate .o files, but there is no
actual limit to how long section names in ELF binaries can be;
they are not stored directly in headers but instead referenced
as an offset in the .shstrtab section, which has all the section
names stored in it.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,fbsder/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr,explora26/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,galak/zephyr,explora26/zephyr,rsalveti/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,runchip/zephyr-cc3220,mbolivar/zephyr,ldts/zephyr,kraj/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,punitvara/zephyr,fbsder/zephyr,fbsder/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,rsalveti/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,GiulianoFranchetto/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,rsalveti/zephyr,Vudentz/zephyr,mbolivar/zephyr,Vudentz/zephyr,kraj/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyriot/zephyr