xfail libgomp.c/declare-variant-4-{fiji,gfx803}.c

Since r14-4734-g56ed1055b2f40ac162ae8d382280ac07a33f789f, GCC no longer
builds the Fiji (alias gfx803) libraries by default as support for it was
removed in ROCm 4.0 and will be removed in LLVM 18.

Thus, unless gfx803 is explicitly enabled, the following testcases will
fail to link as libgomp is not available for Fiji. Hence, this commit
xfails those testcases.

libgomp/ChangeLog:

	* testsuite/libgomp.c/declare-variant-4-fiji.c: Xfail as fiji
	support is no longer enabled by default.
	* testsuite/libgomp.c/declare-variant-4-gfx803.c: Likewise.

Signed-off-by: Tobias Burnus <tburnus@baylibre.com>