[Darwin, PPC, testsuite] Fix pr80125 testcase for Darwin.

[Darwin, PPC, testsuite] Fix pr80125 testcase for Darwin.

Darwin (unlike most of the members of the PowerPC port family)
defaults to signed chars, so the test was failing to compile with
a "mismatched parameters" error.

2019-06-23  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.target/powerpc/pr80125.c (foo): Use an unsigned char
	vector explicitly for the vec_perm.

From-SVN: r272605