Use __NEWLIB__ instead of __SWITCH__ and __CYGWIN__ for alloca.h-detection (#3070)

Cygwin uses newlib, so it's covered by the __NEWLIB__ check.
You can see how it defines __NEWLIB__ here: https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=newlib/configure.in#l453
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