Keep previous cmake arguments when using buildstandalonegc (dotnet/coreclr#12108)

Keep previous cmake arguments when using buildstandalonegc (dotnet/coreclr#12108)

Currently, using buildstandalonegc makes ./build.sh discard any
previous values of __cmakearg set explicitly or implicitly. Preserve
those arguments.

Without this fix, the following invocations behave differently:

    $ ./build.sh ignorewarnings buildstandalonegc verbose

    $ ./build.sh buildstandalonegc ignorewarnings verbose

Commit migrated from https://github.com/dotnet/coreclr/commit/d2f9e21c69e70f1d0edac2392ff95d4670edff6e