[build] Fix sanitizer build on Mac.

It seems on Linux linker parses -usymbol as -u symbol,
but the same does not get parsed on Mac - it interprets
the whole thing as a name of a flag and gets confused.

Fixes https://github.com/dart-lang/sdk/issues/61377

TEST=tried building with TSAN on mac
R=aam@google.com

Change-Id: I75e386813621f6e15562b5d8386f2c4441773970
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446301
Commit-Queue: Alexander Aprelev <aam@google.com>
Auto-Submit: Slava Egorov <vegorov@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

build/config/sanitizers/BUILD.gn

@@ -15,7 +15,7 @@ group("deps") {
 }
 
 config("sanitizer_options_link_helper") {
-  ldflags = [ "-Wl,-u_sanitizer_options_link_helper" ]
+  ldflags = [ "-Wl,-u,_sanitizer_options_link_helper" ]
 }
 
 source_set("options_sources") {

build/sanitizers/sanitizer_options.cc

@@ -8,7 +8,7 @@
 // Functions returning default options are declared weak in the tools' runtime
 // libraries. To make the linker pick the strong replacements for those
 // functions from this module, we explicitly force its inclusion by passing
-// -Wl,-u_sanitizer_options_link_helper
+// -Wl,-u,_sanitizer_options_link_helper
 extern "C" void _sanitizer_options_link_helper() {}
 
 // The callbacks we define here will be called from the sanitizer runtime, but