Abstract deprecated and selectany for GCC and remove LLVM'ism where possible (#22662)

Abstract deprecated and selectany for GCC and remove LLVM'ism where possible (#22662)

* Abstract selectany

* Fix initializer element is not constant

src/corefx/System.Globalization.Native/pal_icushim.c:58:34: error: initializer element is not constant
 static const int MaxICUVersion = MinICUVersion + 20;

* Enable ms extensions

* Apply LLVM patterns to GCC

* Remove deprecated function

* Fix const conversion error

src/corefx/System.Globalization.Native/pal_calendarData.c:390:16: warning: passing argument 1 of ‘ures_close_ptr’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
ures_close(erasResBundle);

src/corefx/System.Globalization.Native/pal_calendarData.c:419:22: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
char* name = GetCalendarName(calendarId);

* Remove old compiler option