Miscellaneous emx.h cleanups
Use the implementation of i386_emx_asm_output_aligned_decl_common in emx.c rather than the definition in emx.h
Add definition of ASM_OUTPUT_DEF_FROM_DECLS from winnt.
This implements the `alias' attribute, keeping any stdcall or fastcall decoration.
reinstated TARGET_MANGLE_DECL_ASSEMBLER_NAME  definition in emx.h