Fix bootstrap on 32-bit SPARC/Solaris

The 'U' constraint cannot be used with LRA.

gcc/
	PR target/113952
	PR target/117168
	* config/sparc/constraints.md ('U'): Delete.
	* config/sparc/sparc.md (*movdi_insn_sp32): Remove U alternatives.
	(*movdf_insn_sp32): Likewise.
	(*mov<VM64:mode>_insn_sp32): Likewise.
	* doc/md.texi (SPARC constraints): Remove entry for 'U'.