Web: Generalize _EXTENSIONS* in SPIR-V back-end.

About 50 fewer #ifdefs.
About 14K smaller.
Note, the base size is ill-defined due to optimizer settings (size vs. performance),
compression, and target architecture.  Some recent %'s are accidentally reported as
3X the real savings.  Early %'s were accurate.  What matters though is that each
step got worthwhile gains, and what the final size ends up being.

glslang/MachineIndependent/glslang.y

@@ -1239,10 +1239,12 @@ layout_qualifier_id
 
 precise_qualifier
     : PRECISE {
+#ifndef GLSLANG_WEB
         parseContext.profileRequires($$.loc, ECoreProfile | ECompatibilityProfile, 400, E_GL_ARB_gpu_shader5, "precise");
         parseContext.profileRequires($1.loc, EEsProfile, 320, Num_AEP_gpu_shader5, AEP_gpu_shader5, "precise");
         $$.init($1.loc);
         $$.qualifier.noContraction = true;
+#endif
     }
     ;