Implement gcc_struct attribute

[pcc]

Bugzilla Link	24383
Version	unspecified
OS	Linux
CC	@erichkeane

Extended Description

This attribute does the opposite of ms_struct (it forces GCC-style bitfield layout in MS mode) so it should hopefully be relatively easy to implement.

[erichkeane]

I investigated this briefly. The Linux implementation is quite straight forward, however the MS RecordLayout functionality doesn't have anything to do this. In fact, -mno-ms-bitfields I don't believe works for Windows targets.