Keep previous cmake arguments when using buildstandalonegc (#12108)
Keep previous cmake arguments when using buildstandalonegc (#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
mmitche/coreclr,rartemev/coreclr,cshung/coreclr,parjong/coreclr,cydhaselton/coreclr,hseok-oh/coreclr,ragmani/coreclr,qiudesong/coreclr,kyulee1/coreclr,tijoytom/coreclr,krytarowski/coreclr,sagood/coreclr,YongseopKim/coreclr,ragmani/coreclr,parjong/coreclr,JosephTremoulet/coreclr,cshung/coreclr,wateret/coreclr,russellhadley/coreclr,poizan42/coreclr,russellhadley/coreclr,russellhadley/coreclr,hseok-oh/coreclr,dpodder/coreclr,krk/coreclr,JosephTremoulet/coreclr,YongseopKim/coreclr,wateret/coreclr,wtgodbe/coreclr,rartemev/coreclr,qiudesong/coreclr,kyulee1/coreclr,krk/coreclr,kyulee1/coreclr,parjong/coreclr,ragmani/coreclr,mmitche/coreclr,qiudesong/coreclr,mskvortsov/coreclr,botaberg/coreclr,mskvortsov/coreclr,sagood/coreclr,kyulee1/coreclr,wateret/coreclr,parjong/coreclr,cydhaselton/coreclr,pgavlin/coreclr,hseok-oh/coreclr,krk/coreclr,YongseopKim/coreclr,botaberg/coreclr,dpodder/coreclr,tijoytom/coreclr,krytarowski/coreclr,JosephTremoulet/coreclr,rartemev/coreclr,JonHanna/coreclr,rartemev/coreclr,tijoytom/coreclr,wateret/coreclr,poizan42/coreclr,JosephTremoulet/coreclr,poizan42/coreclr,cshung/coreclr,JonHanna/coreclr,gkhanna79/coreclr,wateret/coreclr,russellhadley/coreclr,mmitche/coreclr,botaberg/coreclr,JonHanna/coreclr,cydhaselton/coreclr,gkhanna79/coreclr,qiudesong/coreclr,poizan42/coreclr,gkhanna79/coreclr,kyulee1/coreclr,parjong/coreclr,rartemev/coreclr,cydhaselton/coreclr,JonHanna/coreclr,krytarowski/coreclr,wtgodbe/coreclr,gkhanna79/coreclr,mskvortsov/coreclr,botaberg/coreclr,botaberg/coreclr,hseok-oh/coreclr,yizhang82/coreclr,cydhaselton/coreclr,pgavlin/coreclr,qiudesong/coreclr,cshung/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,parjong/coreclr,sagood/coreclr,JonHanna/coreclr,hseok-oh/coreclr,poizan42/coreclr,yizhang82/coreclr,gkhanna79/coreclr,mmitche/coreclr,hseok-oh/coreclr,pgavlin/coreclr,sagood/coreclr,wateret/coreclr,YongseopKim/coreclr,wtgodbe/coreclr,JosephTremoulet/coreclr,AlexGhiondea/coreclr,AlexGhiondea/coreclr,dpodder/coreclr,krytarowski/coreclr,rartemev/coreclr,yizhang82/coreclr,pgavlin/coreclr,krk/coreclr,ragmani/coreclr,ruben-ayrapetyan/coreclr,qiudesong/coreclr,AlexGhiondea/coreclr,YongseopKim/coreclr,cshung/coreclr,russellhadley/coreclr,wtgodbe/coreclr,YongseopKim/coreclr,mskvortsov/coreclr,JosephTremoulet/coreclr,krytarowski/coreclr,cshung/coreclr,mmitche/coreclr,ruben-ayrapetyan/coreclr,tijoytom/coreclr,gkhanna79/coreclr,tijoytom/coreclr,yizhang82/coreclr,krk/coreclr,sagood/coreclr,wtgodbe/coreclr,krytarowski/coreclr,poizan42/coreclr,tijoytom/coreclr,ragmani/coreclr,ruben-ayrapetyan/coreclr,botaberg/coreclr,AlexGhiondea/coreclr,ruben-ayrapetyan/coreclr,kyulee1/coreclr,sagood/coreclr,AlexGhiondea/coreclr,AlexGhiondea/coreclr,russellhadley/coreclr,mmitche/coreclr,mskvortsov/coreclr,dpodder/coreclr,pgavlin/coreclr,wtgodbe/coreclr,mskvortsov/coreclr,pgavlin/coreclr,yizhang82/coreclr,ragmani/coreclr,ruben-ayrapetyan/coreclr,JonHanna/coreclr,dpodder/coreclr,cydhaselton/coreclr,krk/coreclr,dpodder/coreclr