[compiler-rt][builtins][sanitizers] Update compiler-rt test cases for

compatibility with system's toolchain

This patch aims to:
- Guard ompiler-rt/test/builtins/Unit/compiler_rt_logb_test.c with macros, so
the test runs on GLIBC versions >= 2.23. This is because the test relies on
comparing its computed values to libm. Oolder versions might not compute to the
same value as the compiler-rt value.
- Update compiler-rt/test/sanitizer_common/TestCases/Posix/getpw_getgr.cc
so that std::string is not used, since false positives may be detected.

Differential Revision: https://reviews.llvm.org/D60644

llvm-svn: 359606