Add configure flag to enable gnu hash style by default.

Add configure flag to enable gnu hash style by default.

ld/
	* configure.ac: Add --enable-default-hash-style option.
	* ldmain.c (main): Set link_info.emit_hash to DEFAULT_EMIT_SYSV_HASH.
	Set link_info.emit_gnu_hash to DEFAULT_EMIT_GNU_HASH.
	* configure: Regenerate.
	* config.in: Regenerate.

gold/
	* configure.ac: Add --enable-default-hash-style option.
	* options.h (hash_style): Use DEFAULT_HASH_STYLE as default value.
	* configure: Regenerate.
	* config.in: Regenerate.