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

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

The gen_directive_tests script uses a bash shebang so == does work, but
there's no reason this script can't just use the more portable form
anyway.

	PR bootstrap/105831

gcc/ChangeLog:

	* config.gcc: Use = operator instead of ==.

gcc/testsuite/ChangeLog:

	* gcc.test-framework/gen_directive_tests: Use = operator instead
	of ==.