PR modula2/116073 invalid rtl sharing compiling FileSystem.mod caused by ext-dce

The bug fixes to PR modula2/118010 and PR modula2/118183 uncovered a bug
in the procedure interface to lseek which uses SYSTEM.COFF_T rather than
SYSTEM.CSSIZE_T.  This patch sets the default size for COFF_T to the same
as CSSIZE_T.

gcc/ChangeLog:
	PR modula2/118010
	PR modula2/118183
	PR modula2/116073
	* doc/gm2.texi (-fm2-file-offset-bits=): Change the default size
	description to CSSIZE_T.
	Add COFF_T to the list of data types exported by SYSTEM.def.

gcc/m2/ChangeLog:
	PR modula2/118010
	PR modula2/118183
	PR modula2/116073
	* gm2-compiler/M2Options.mod (OffTBits): Assign to 0.
	* gm2-gcc/m2type.cc (build_m2_specific_size_type): Ensure that
	layout_type is called before returning c.
	(build_m2_offt_type_node): If GetFileOffsetBits returns 0 then
	use the type size of ssize_t.

gcc/testsuite/ChangeLog:

	PR modula2/118010
	PR modula2/118183
	PR modula2/116073
	* gm2/pim/run/pass/printtypesize.mod: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>