testsuite/lib/libgomp.exp: compile with -fdiagnostics-plain-output

testsuite/lib/libgomp.exp: compile with -fdiagnostics-plain-output

libgomp.exp added -fno-diagnostics-show-caret and -fdiagnostics-color=never
as 'additional_flags' for compilation. However, it turned out that this now
is insufficient as the [...] part of diagnostics have a hyperlink URL.

Solution: Use the -fdiagnostics-plain-output flag instead, added in commit
r11-2701-g129a1319c0ab73. This flag currently implies the following flags:
   -fno-diagnostics-show-caret
   -fno-diagnostics-show-line-numbers
   -fdiagnostics-color=never
   -fdiagnostics-urls=never
   -fdiagnostics-path-format=separate-events
   -fdiagnostics-text-art-charset=none
   -fno-diagnostics-show-event-links

libgomp/ChangeLog:

	* testsuite/lib/libgomp.exp (libgomp_init): Add
	-fdiagnostics-plain-output to additional_flags; remove
	-fno-diagnostics-show-caret and -fdiagnostics-color=never.