M68K: avoid quadratic slowdlow in label alignment check

M68K: avoid quadratic slowdlow in label alignment check

Before the change tc-m68k maintained a list of seen labels.
Alignment check traversed label list to resolve symbol to label.
This caused quadratic slowdown as each symbol was checked against
each label. Worst affected files are the ones built with debugging
enabled as DWARF generates many labels.

The change embeds auxiliary label information right into symbol using
TC_SYMFIELD_TYPE.

Before the change test from PR 29058 did not finish in 10 minutes. After
the change it finishes in 2 seconds.

gas/ChangeLog:

	PR 29058
	* config/tc-m68k.h (TC_SYMFIELD_TYPE): define as m68k_tc_sy.
	* config/tc-m68k.c (m68k_frob_label): Use TC_SYMFIELD_TYPE to
	store label information.