mac: When running dsymutil, run fewer links in parallel.

dsymutil (which runs as part of the link in official builds by default)
needs a lot of memory. Account for that.

Bug: 1028112
Change-Id: I555c265c1e6313327cecb5ab292063f04e4ff30b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007648Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732829}

build/toolchain/concurrent_links.gni

@@ -8,6 +8,7 @@
 assert(current_toolchain == default_toolchain)
 
 import("//build/config/compiler/compiler.gni")
+import("//build/config/mac/symbols.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
@@ -38,7 +39,11 @@ if (concurrent_links == -1) {
   } else if (is_win) {
     _args = [ "--mem_per_link_gb=5" ]
   } else if (is_mac) {
-    _args = [ "--mem_per_link_gb=4" ]
+    if (enable_dsyms) {
+      _args = [ "--mem_per_link_gb=12" ]
+    } else {
+      _args = [ "--mem_per_link_gb=4" ]
+    }
   } else if (is_android && !is_component_build && symbol_level == 2) {
     # Full debug symbols require large memory for link.
     _args = [ "--mem_per_link_gb=25" ]