contrib: Fix nonportable shell syntax in "test" and "[" commands [PR105831]

POSIX sh does not support the == for string comparisons, use = instead.

These contrib scripts all use a bash shebang so == does work, but
there's no reason they can't just use the more portable form anyway.

	PR bootstrap/105831

contrib/ChangeLog:

	* bench-stringop: Use = operator instead of ==.
	* repro_fail: Likewise.

contrib/reghunt/ChangeLog:

	* bin/reg-hunt: Use = operator instead of ==.