Extend the targets of debug_fission with ThinLTO

ThinLTO requires -gsplit-dwarf in ldflags.
The flag was passed only in Android.

Pass -gsplit-dwarf to ldflags with all targets except NaCL and Apple.

Bug: chromium:1158215
Change-Id: Ica36d0ed0cb3382792b3f21b30af56638faf24ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618780Reviewed-by: Bob Haarman <inglorion@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Denis Nikitin <denik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844347}

build/config/compiler/BUILD.gn

@@ -2339,8 +2339,9 @@ config("symbols") {
     asmflags = cflags
     ldflags = []
 
-    # TODO(agrieve): Not sure why this is Android-specific.
-    if (use_debug_fission && use_thin_lto && is_android) {
+    # Split debug info with all thinlto builds except nacl and apple.
+    # thinlto requires -gsplit-dwarf in ldflags.
+    if (use_debug_fission && use_thin_lto && !is_nacl && !is_apple) {
       ldflags += [ "-gsplit-dwarf" ]
     }
 
@@ -2415,7 +2416,9 @@ config("minimal_symbols") {
       cflags += [ "-fdebug-info-for-profiling" ]
     }
 
-    # Note: -gsplit-dwarf implicitly turns on -g2 with clang, so don't pass it.
+    # Note: debug_fission is no-op with symbol_level=1 since all -g1 debug_info
+    # will stay in the executable.
+
     asmflags = cflags
   }
 }