Runtime symbols referenced by MemorySanitizer instrumentation (e.g.
__msan_warning_noreturn) reside in the executable and are exported from
it (i.e. __attribute__((visibility("default"))). They can be
resolved using dlsym(RTLD_DEFAULT). This works for either an executable
or shared library using Reactor.

Note that this differs from our regular symbol Resolver approach which
lists pairs of symbol names and addresses. There is no public header
which declares these MSan symbols, and if we list symbols only available
in a newer version of LLVM's compiler-rt, the executable won't link or
the shared library won't load.

This change has no effect at this point. A later change will enable
MemorySanitizer intrumentation for Reactor routines.

Bug: b/155148722
Change-Id: I4601e499a2bf7073dbdd68679b74518500ebe217
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49848Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>