build/roms: Fix building for ARMv7 and AArch64 boards

The code that compiles coreboot crossgcc changes the working directory
to the coreboot directory, and the following code cannot find the lbmk
scripts that it needs to run. Compile ARMv7 and AArch64 cross compilers
in a subshell like in the x86 case so the rest of the script can work.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>