toolchain/wrapper: fix potential bug in foreach loop

In Makefile, the comma ',' is used to separate the arguments passed to
functions, so we should not be allowed to use straight commas in strings
we want to expand.

For the toolchain wrapper, we need to transform a list:
    -mfoo -mbar -mbuz

into something acceptable for a C array assignment:
    "-mfoo", "-mbar", "-mbuz",

So, we use a $(foreach ...) loop for that. However, we do have a
straight comma in there.

It does not cause any issue in practice, since $(foreach) is a make
builtin function that accepts three and only three parameters.

However, this is not sane.

Change the straight comma to the usual $(comma) expansion, like we would
do for a call to any other function.

At the same time, make the code a bit easier to read, by first creating
the transformed list, and then creating the define.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>