[compiler-rt] Fix build of Sanitizer-Test_Nolibc after D80648

Running ninja check-sanitizer fails for after that patch (commit
058f5f6f) with the following error:

libRTSanitizerCommon.test.nolibc.x86_64.a(sanitizer_posix.cpp.o): In
function `__sanitizer::GetNamedMappingFd(char const*, unsigned long,
int*)':
..../llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp:358:
undefined reference to `fcntl'
clang-12: error: linker command failed with exit code 1 (use -v to see
invocation)

This patch works around the problem by only calling fcntl if O_CLOEXEC
is not defined.

Reviewed By: plopresti

Differential Revision: https://reviews.llvm.org/D85114