Usage of __THROW in omrsig.h/cpp extern C code

[Deigue]

Wanted to create an issue to separate out the problem raised during review of #7320

Not directly related to the changed being made, but a cause for concern here in https://github.com/eclipse-omr/omr/blob/master/include_core/omrsig.h , are the uses of __THROW seemingly within extern C code,

Their usages also corresponding in https://github.com/eclipse-omr/omr/blob/master/omrsigcompat/omrsig.cpp as result could be invalid.

Further there is also #7320 (comment) , referring to

omr/omrsigcompat/omrsig.cpp

Line 45 in c5dbec1

extern "C" {

. The need for the definition of sighandler itself to be within extern C.

I had an initial attempt at removing these but was facing errors/issues after having undone the macro definitions/usages.

Creating this issue to piece this out and investigate separately and try to get the original PR #7320 merged asap, as it has been open for a long period and is critical for the Open XL build enablement.