[PSEH] Add implementation for GCC amd64

[PSEH] Add implementation for GCC amd64

Also, put include directory next to the library and use
target_include_directories(.. INTERFACE ..) to get this right.
This is because :
 - Having includes & implementation in two different places buggers me
 - This makes sure that there is no "if it compiles everything is fine" behaviour from anyone
   because now even static libraries need it for GCC amd64 build
Also add __USE_PSEH2__ define for the non SEH-aware compilers out there and use it in a few headers
where we define macros involving __try