aarch64: Add Cygwin and MinGW environments for AArch64
Define Cygwin and MinGW environment such as types, SEH definitions,
shared libraries, etc.
gcc/ChangeLog:
* config.gcc: Add Cygwin and MinGW difinitions.
* config/aarch64/aarch64-protos.h
(mingw_pe_maybe_record_exported_symbol): Declare functions
which are used in Cygwin and MinGW environment.
(mingw_pe_section_type_flags): Likewise.
(mingw_pe_unique_section): Likewise.
(mingw_pe_encode_section_info): Likewise.
* config/aarch64/cygming.h: New file.