[Darwin, X86, testsuite] Add xfails for PR90396.

[Darwin, X86, testsuite] Add xfails for PR90396.

We don't have support for -mcmodel={medium, large, kernel} so don't
expect tests for those things to work.

For now mark them as xfail where possible and skip where that isn't.
These changes will be logged onto the PR and therefore can be backed
out when the facility is implemented.

2019-09-08  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline.
	2019-06-01  Iain Sandoe  <iain@sandoe.co.uk>

	PR target/90698
	* gcc.target/i386/pr49866.c: XFAIL for Darwin.
	* gcc.target/i386/pr63538.c: Likewise.
	* gcc.target/i386/pr61599-1.c: Skip for Darwin.

From-SVN: r275496