Allow specifying compiler mode options in buildfile

Now the configured mode options are appended to buildfile-specified (which
must be specified before loading the guess module).

In particular, this ability to specify the compiler mode in a buildfile is
useful in embedded development where the project may need to hardcode things
like -target, -nostdinc, etc. For example:

cxx.std = 20
cxx.mode = -target riscv32-unknown-unknown -nostdinc

using cxx

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