[bazel] Move _mask_rom_interrupt_vector_c to .vectors to eliminate padding

[alphan]

I had some time to look more into the size difference between the binaries built using meson vs bazel, specifically why mask_rom_exception_handler() is placed at the start of the .crt section when using bazel. This introduces extra padding since _mask_rom_interrupt_vector_c is aligned to 256 bytes.

I think, the reason why we don't have this issue in meson is because mask_rom_start.S appears before mask_rom.c. Reordering them in the srcs of opentian_rom_binary() in sw/device/silicon_creator/mask_rom/BUILD fixes the issue but would be very fragile (also not allowed by the buildifier).

I was thinking about updating the linker file and mask_rom.h to put the functions defined in c after asm definitions but realized that we can actually place _mask_rom_interrupt_vector_c into .vectors instead of .crt without any overhead since the c interrupt vector is aligned to 256 bytes as well, i.e. the comment removed in this PR doesn't seem to be accurate.

mask_rom_fpga_cw310.elf.s: before, after

This PR also reduces the size of mask_rom_fpga_cw310.bin from 31832 bytes to 31640 bytes (-192 bytes)

Related to #12069 (@timothytrippel).

Signed-off-by: Alphan Ulusoy [email protected]