Only write CMakeLists.txt for normal variants

Modules may have multiple variants for each architecture, and
SOONG_GEN_CMAKEFILES=1 was generating CMakeLists.txt for all of
them, overwriting the first normal variant with the last (and
likely least normal) variant.

Keep a map of generated CMakeLists.txt files, and only generate
each one once.

Bug: 72283227
Test: m SOONG_GEN_CMAKEFILES=1
Change-Id: If31087264d2809088fe36965b9a0ff0f44089852