[clang, test] Fix use of undef FileCheck var

Clang test CodeGen/libcalls.c contains CHECK-NOT directives using a
variable defined in a CHECK directive with a different prefix never
enabled together, therefore causing the variable to be undefined in that
CHECK-NOT.

The intent of the test is to check that some declaration do not have the
same attribute as when compiling the test without -fmath-errno. This
commits instead changes all CHECK-NOT to CHECK directive, checking that
they all use the same attribute. It also adds an extra CHECK for that
prefix to check the expected attributes these functions should have when
compiling with -fmath-errno.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D99898