[sanitizer_common] Define internal_usleep on Solaris

The Solaris/amd64 buildbot
<https://lab.llvm.org/staging/#/builders/101/builds/2845> has recently been
broken several times, at least one of those remains unfixed:

  [63/446] Generating Sanitizer-x86_64-Test
  [...]
  Undefined			first referenced
   symbol  			    in file
  _ZN11__sanitizer15internal_usleepEy /opt/llvm-buildbot/home/solaris11-amd64/clang-solaris11-amd64/stage1/projects/compiler-rt/lib/sanitizer_common/tests/libRTSanitizerCommon.test.x86_64.a(sanitizer_common.cpp.o)
  ld: fatal: symbol referencing errors

Thist patch fixes it by defining the missing `internal_usleep`.

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

Differential Revision: https://reviews.llvm.org/D105878