De-duplicate the retro definitions?

[Artoria2e5]

We have a lot of duplication in the "retro" arch definitions when it comes to OPTI args; just grepping for # Retro: should reveal them all. The consensus seems to be the following, more or less:

# Retro: Overriding mainline definitions, and take more interest in reducing code size.
CFLAGS_COMMON_OPTI=('-Os' '-ffunction-sections' '-fdata-sections')
LDFLAGS_COMMON_OPTI=('-Wl,--gc-sections')
# Retro: Also disabling -ftree-vectorization which could potentially enlarge code size.
CFLAGS_GCC_OPTI=('-fira-loop-pressure' '-fira-hoist-pressure')

Maybe we can get a new file called arch/_retro.sh and just do load_strict "$AB/arch/_retro.sh" in these files, where it applies.