[mono] Fix compilation with compilers missing `__has_extension` (#29435)
[mono] Fix compilation with compilers missing `__has_extension` (#29435)

For some obscure reasons, `if defined(__has_extension) && __has_extension(c_static_assert)` wouldn't be able to guard against the absence of `__has_entension` with some compilers. To fix that, we just make sure it is always defined, even if it will by default always return `0`.
shimingsg/corefx,ViktorHofer/corefx,Jiayili1/corefx,ericstj/corefx,ptoonen/corefx,shimingsg/corefx,Jiayili1/corefx,wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,mmitche/corefx,wtgodbe/corefx,ericstj/corefx,ViktorHofer/corefx,BrennanConroy/corefx,ViktorHofer/corefx,ericstj/corefx,ericstj/corefx,mmitche/corefx,ptoonen/corefx,ptoonen/corefx,ViktorHofer/corefx,mmitche/corefx,mmitche/corefx,ericstj/corefx,wtgodbe/corefx,ptoonen/corefx,shimingsg/corefx,ericstj/corefx,ViktorHofer/corefx,Jiayili1/corefx,Jiayili1/corefx,Jiayili1/corefx,wtgodbe/corefx,Jiayili1/corefx,ptoonen/corefx,Jiayili1/corefx,ViktorHofer/corefx,BrennanConroy/corefx,mmitche/corefx,ptoonen/corefx,shimingsg/corefx,ericstj/corefx,ptoonen/corefx,shimingsg/corefx,wtgodbe/corefx,mmitche/corefx,shimingsg/corefx,mmitche/corefx,BrennanConroy/corefx,wtgodbe/corefx,wtgodbe/corefx