thiscall calling convention (-gen gcc only)

[jayrm]

Builds on PR #303 (Add deleting destructor for c++ ABI) to add __thiscall keyword to specify the the thiscall calling convention and add additional tests for 32-bit windows gcc for the c++ ABI.

32-bit windows should be only target in need of the thiscall calling convention.

To allow some flexibility for testing code, fbc does not automatically apply the thiscall calling convention, though this could be changed in a later update. Currently __thiscall must be explicitly specified. For example:

#if defined(__FB_WIN32__) and not defined(__FB_64BIT__)
	#define thiscall __thiscall
#else
	#define thiscall
#endif

extern "c++"
	
type UDT
	value as long
	declare constructor thiscall ()
	declare constructor thiscall ( byval rhs as long )
	declare constructor thiscall ( byref rhs as const UDT )
	declare destructor thiscall ()
end type

end extern

Additionally, the fbc command line option -z no-thiscall is added to effectively disable (ignore) the __thiscall keyword to assist in moving between use and testing of the thiscall calling convention.

__thiscall is only implemented for the gcc backend. The gas backend needs many internal changes to support passing arguments in registers to support the thiscall calling convention. It seems prudent to build up the test cases in gcc before modifying the gas backend possible in a later update.

Some tests are disabled with a ENABLE_CHECK_BUGS guard because:

• __thiscall not supported by gas backend
• some tests have implementation for some simple member operators but this is not yet supported by fbc, see PR fbc: add static operator to user defined types #149 Add Static Operator to User defined types