darwin, x86, testsuite - Amend popcnt match string for Darwin.

Darwin uses a different spelling for popcnt (popcnt instead of popcntw).
Check for this in the test.

2019-05-25  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.target/i386/pr59874-3.c: Use the spelling of popcnt
	expected for Darwin.

From-SVN: r271623