Fix implicit-fallthrough warnings in tst-setjmp.c.

Building the testsuite with -Wextra (together with
-Wno-cast-function-type -Wno-clobbered -Wno-expansion-to-defined
-Wno-missing-field-initializers -Wno-old-style-declaration
-Wno-shift-negative-value -Wno-sign-compare -Wno-type-limits
-Wno-unused-parameter, which reflect the set of -Wextra warnings for
which glibc itself is not currently clean on x86_64) showed up
implicit-fallthrough warnings in tst-setjmp.c.  Those warnings appear
to be false positives, arising from a function "jump" that calls
longjmp not itself being marked as noreturn; thus, this patch adds the
noreturn marking to that function to fix the warnings.

Tested for x86_64.

	* setjmp/tst-setjmp.c (jump): Use __attribute__ ((__noreturn__)).