Fix incorrect handling of STT_COMMON symbols in shared libraries.

Fix incorrect handling of STT_COMMON symbols in shared libraries.

The gABI allows STT_COMMON symbols to appear in executables and shared
objects, so that the dynamic loader can resolve commons across modules.
When reading a shared object, however, an STT_COMMON symbol should be
treated as a regular definition at link time.

In a relocatable object, the gABI requires that any STT_COMMON symbols
must also be defined in the special SHN_COMMON section (which we extend
to include target-specific small and large common sections). Thus,
there is no need for gold to treat STT_COMMON symbols as common unless
the st_shndx field is also set to a common section.

gold/
	PR gold/18288
	* resolve.cc (symbol_to_bits): Remove type parameter; adjust all
	callers. Don't use STT_COMMON to check for common symbols.
	(Symbol_table::resolve): Warn if relocatable object has STT_COMMON
	symbol that's not in a common section.
	* symtab.h (Symbol::is_common): Don't use STT_COMMON to check for
	common symbols.