Convert spvc build flag to a runtime toggle

Also moves some of the spirv_cross code into the main library that was
feature guarded, since spvc requires it.

BUG=dawn:281

Change-Id: I482d1d5a5c851956d3815bad90665c52a1ea15bb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13860
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

scripts/dawn_features.gni

@@ -44,10 +44,6 @@ declare_args() {
   # GLSL/HLSL/MSL compiler. This implicitly pulls in the GLSL
   # compiler, since it is a sub-class of if.
   dawn_enable_cross_reflection = false
-
-  # Enables using spvc for accessing SPIR-V toolchain, instead of
-  # directly accessing it in Dawn.
-  dawn_use_spvc = false
 }
 
 # GN does not allow reading a variable defined in the same declare_args().