Compile CanvasKit without -fPIC, for latest emscripten compatibility.

Due to an issue in emscripten's latest-upstream compiler (link below),
Skia's libraries can't be built using -fPIC. This change allows users
to opt-out of using -fPIC during compilation, and sets compile.sh to
opt-out accordingly.
https://github.com/emscripten-core/emscripten/issues/8995
https://github.com/emscripten-core/emscripten/issues/9317

Change-Id: I25b6e185b58ab1ba8263f2e2f450591f6025f62c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243317
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

gn/BUILD.gn

@@ -9,6 +9,7 @@ declare_args() {
   extra_cflags_c = []
   extra_cflags_cc = []
   extra_ldflags = []
+  use_PIC = true
 
   malloc = ""
 }
@@ -88,10 +89,10 @@ config("default") {
       "$win_vc/Tools/MSVC/$win_toolchain_version/lib/$target_cpu",
     ]
   } else {
-    cflags += [
-      "-fstrict-aliasing",
-      "-fPIC",
-    ]
+    if (use_PIC) {
+      cflags += [ "-fPIC" ]
+    }
+    cflags += [ "-fstrict-aliasing" ]
     cflags_cc += [ "-std=c++14" ]
 
     # The main idea is to slim the exported API, but these flags also improve link time on Mac.

modules/canvaskit/compile.sh

@@ -162,6 +162,7 @@ echo "Compiling bitcode"
   is_component_build=false \
   werror=true \
   target_cpu=\"wasm\" \
+  use_PIC=false \
   \
   skia_use_angle = false \
   skia_use_dng_sdk=false \