cmake: gcc/ld: Fix LTO warnings

Fix the following warnings:
[255/261] Linking C executable zephyr/zephyr_pre0.elf
lto-wrapper: warning: using serial compilation of 8 LTRANS jobs
lto-wrapper: note: see the '-flto' option documentation for more
information
[260/261] Linking C executable zephyr/zephyr.elf
lto-wrapper: warning: using serial compilation of 8 LTRANS jobs
lto-wrapper: note: see the '-flto' option documentation for more
information

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>