[MemProf] Decouple memprof build from COMPILER_RT_BUILD_SANITIZERS

The MemProf compiler-rt support relies on some of the support only built
when COMPILER_RT_BUILD_SANITIZERS was enabled. This showed up in some
initial bot failures, and I addressed those by making the memprof
runtime build also conditional on COMPILER_RT_BUILD_SANITIZERS
(3ed77ecd). However, this resulted in
another inconsistency with how the tests were set up that was hit by
Chromium:
  https://bugs.chromium.org/p/chromium/issues/detail?id=1142191

Undo the original bot fix and address this with a more comprehensive fix
that enables memprof to be built even when COMPILER_RT_BUILD_SANITIZERS
is disabled, by also building the necessary pieces under
COMPILER_RT_BUILD_MEMPROF.

Tested by configuring with a similar command as to what was used in the
failing Chromium configure. I reproduced the Chromium failure, as well
as the original bot failure I tried to fix in
3ed77ecd, with that fix reverted.
Confirmed it now works.

Differential Revision: https://reviews.llvm.org/D90190