Redo gas local symbol support

gas handles local symbols specially in order to save memory, but the
implementation using two separate hash tables is inefficient,
particularly the scheme of duplicating a struct local_symbol when it
needs to be converted to a full struct symbol.  Also, updating symbol
pointers with LOCAL_SYMBOL_CHECK is horrible and has led to some hard
to find bugs.

This changes the implementation to use a single hash table and avoids
another copy of the symbol name in symbol_entry_t.  When converting
local symbols the struct local_symbol memory is reused.  Not only
does that save memory, but there is no need to twiddle symbol pointers
with LOCAL_SYMBOL_CHECK.

Assembling gcc-10 -g -Og gold/powerpc.cc output shows the following:

old:
symbol table hash statistics:
	1371192 searches
	1290398 collisions
	143585 elements
	262139 table size
mini local symbol table hash statistics:
	2966204 searches
	2707489 collisions
	523533 elements
	1048573 table size
523533 mini local symbols created, 140453 converted

new:
symbol table hash statistics:
	2828883 searches
	2453138 collisions
	526665 elements
	1048573 table size
523533 mini local symbols created, 140453 converted

	* symbols.c (struct local_symbol): Add "hash" entry.  Reorder fields.
	Delete union.  Adjust code throughout file.
	(struct symbol): Add "hash", "name" and "x" entries.  Reorder fields.
	Split off some to..
	(struct xsymbol): ..this.  New struct.  Adjust code throughout file
	accessing these fields.
	(struct symbol_entry): Delete.
	(union symbol_entry): New.
	(hash_symbol_entry): Adjust for symbol_entry_t change.
	(symbol_entry_find): Likewise.
	(eq_symbol_entry): Compare hash values too.
	(symbol_entry_alloc): Delete.
	(local_symbol_converted_p, local_symbol_mark_converted): Delete.
	(local_symbol_get_real_symbol, local_symbol_set_real_symbol): Delete.
	(local_hash): Delete.
	(abs_symbol_x, dot_symbol_x): New static var.
	(symbol_init): New function.
	(symbol_create): Rewrite.
	(LOCAL_SYMBOL_CHECK): Delete.  Replace uses throughout with simple
	test of flags.local_symbol.
	(local_symbol_make): Adjust for struct local_symbol changes.
	(local_symbol_convert): Rewrite.  Adjust all callers.
	(symbol_table_insert): Simplify.
	(symbol_clone): Comment on local sym cloning.  Handle split symbol
	struct.
	(get_real_sym): Delete.  Remove all uses.
	(symbol_find_exact_noref): Simplify.
	(resolve_local_symbol): Don't resolve non-locals.
	(S_SET_SEGMENT): Don't special case reg_section.
	(S_SET_NAME): Set both name and bsym->name.
	(symbol_mark_resolved, symbol_resolved_p): Simplify.
	(symbol_symbolS): Update comment.
	(symbol_begin): Don't create local_hash.  Adjust abs_symbol setup.
	(dot_symbol_init): Adjust dot_symbol setup.
	(symbol_print_statistics): Delete local_hash stats.