[sanitizers] Fix internal__exit on Solaris

`TestCases/log-path_test.cpp` currently `FAIL`s on Solaris:

  $ env ASAN_OPTIONS=log_path=`for((i=0;i<10000;i++)); do echo -n $i; done`  ./log-path_test.cpp.tmp
  ==5031==ERROR: Path is too long: 01234567...
  Segmentation Fault (core dumped)

The `SEGV` happens here:

  Thread 2 received signal SIGSEGV, Segmentation fault.
  [Switching to Thread 1 (LWP 1)]
  0x00000000 in ?? ()
  (gdb) where
  #0  0x00000000 in ?? ()
  #1  0x080a1e63 in __interceptor__exit (status=1)
      at /vol/gcc/src/llvm/llvm/local/projects/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:3808
  #2  0x08135ea8 in __sanitizer::internal__exit (exitcode=1)
      at /vol/gcc/src/llvm/llvm/local/projects/compiler-rt/lib/sanitizer_common/sanitizer_solaris.cc:139

when `__interceptor__exit` tries to call `__interception::real__exit` which
is `NULL` at this point because the interceptors haven't been initialized yet.

Ultimately, the problem lies elsewhere, however: `internal__exit` in
`sanitizer_solaris.cpp` calls `_exit` itself since there doesn't exit a
non-intercepted version in `libc`.  Using the `syscall` interface instead
isn't usually an option on Solaris because that interface isn't stable.
However, in the case of `SYS_exit` it can be used nonetheless: `SYS_exit`
has remained unchanged since at least Solaris 2.5.1 in 1996, and this is
what this patch does.

Tested on `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D88404