arch: common: Force linker to fill empty spaces in rom_start with 0x00

LLVM LLD fills empty spaces (created using ALIGN() or moving the
location counter) in executable segments with TrapInstr pattern,
e.g. for ARM the TrapInstr pattern is 0xd4d4d4d4. GNU LD fills
empty spaces with 0x00 pattern.

We may want to have some section (e.g. rom_start) filled with 0x00,
e.g. because MCU can interpret the pattern as a configuration data.

Signed-off-by: Patryk Duda <pdk@semihalf.com>