This enables native unwind semantics with longjmp on
win64 by putting an entry into the .pdata section for
each compiled fuction.
Also, the function now use a fixed stack and store arguments
into X(%rsp) rather than using push.
(Because GNU's alloca.h unconditionally #undef's alloca)
Also, remove gcc specific sections in headers. and
instead change tests such that gcc does not use them.