Automerge: Make sure all matrix options are run in libc premerge testing (#146162)

llvm/llvm-project#126315 changes the premerge
testing to build in Debug, Release, and MinSizeRel configs, as opposed
to only MinSizeRel, but for some reason the matrix is not returning all
options now. Fix so that it actually builds the intended configs.

Per discussion in
[146161](llvm/llvm-project#146161), only test
all three configs on Linux x86_64. Other OS's and configurations will
build in Debug mode only.

Also fix a cmake variable and remove outdated comments.

Bug: llvm/llvm-project#146161

Author: amykhuang