scripts: Add the ISA to the path of included libraries

for GCC ARM Embedded

When compiling and linking Zephyr with GCC ARM Embedded,
the path to the libraries to be linked needs to be provided
explicitely for the correct variant to be linked in.

JIRA: ZEP-377
Change-Id: I745aa45c7dde12f1cc5c8ea4380b904e086ed94d
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>