Fix "obvious" fall-through warnings

Fix "obvious" fall-through warnings

This patch fixes the subset of -Wimplicit-fallthrough warnings that I
considered obvious.  In most cases it was obvious from context that
falling through was desired; here I added the appropriate comment.  In
a couple of cases it seemed clear that a "break" was missing.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* riscv-tdep.c (riscv_isa_xlen): Add fall-through comment.
	* utils.c (can_dump_core) <LIMIT_CUR>: Add fall-through comment.
	* eval.c (fetch_subexp_value) <MEMORY_ERROR>: Add fall-through
	comment.
	* d-valprint.c (d_val_print) <TYPE_CODE_STRUCT>: Add fall-through
	comment.
	* coffread.c (coff_symtab_read) <C_LABEL>: Add fall-through
	comment.