wasm: Change compiler flags to target "upstream" llvm

As of version 1.39, Emscripten now uses upstream llvm instead of
the "fastcomp" fork. As a result of this em++ no longer builds
via the asm.js text format, and the performance issues related
to text parsing are gone.

Reducing the binary size for debug builds is no longer needed,
and we can fall back to using the default build flags.

This effectively makes 1.39.x the minimum Emscripten version for
Qt 5.15. The "-fastcomp" SDK variant is not supported.

Change-Id: Id66ef77294a9accd1b103e34785458d71dc188b1
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

mkspecs/wasm-emscripten/qmake.conf

@@ -51,34 +51,8 @@ EMCC_COMMON_LFLAGS_DEBUG = \
     # -s SOCKET_DEBUG \ #print out socket,network data transfer
     -s GL_DEBUG=1
 
-# Set up debug/optimization flags
-QMAKE_CXXFLAGS_RELEASE -= -O2
-QMAKE_CFLAGS_RELEASE   -= -O2
-equals(WASM_OBJECT_FILES, 1) {
-    QMAKE_LFLAGS_DEBUG     += -g
-
-    QMAKE_CXXFLAGS_RELEASE += -O3
-    QMAKE_CFLAGS_RELEASE   += -O3
-    QMAKE_LFLAGS_RELEASE   += -O3
-    QMAKE_CFLAGS_OPTIMIZE  += -O3
-    QMAKE_CFLAGS_OPTIMIZE_FULL += -Oz
-} else {
-    # Practical debugging setup:
-    #    "-g4" preserves function names for stack traces
-    #    "-Os" produces reasonably sized binaries
-    QMAKE_CFLAGS_DEBUG     -= -g
-    QMAKE_CXXFLAGS_DEBUG   -= -g
-    QMAKE_CFLAGS_DEBUG     += -Os -g4
-    QMAKE_CXXFLAGS_DEBUG   += -Os -g4
-    QMAKE_LFLAGS_DEBUG     += -Os -g4
-
-    QMAKE_CXXFLAGS_RELEASE += -O3
-    QMAKE_CFLAGS_RELEASE   += -O3
-    QMAKE_LFLAGS_RELEASE   += -O3
-    QMAKE_CFLAGS_OPTIMIZE  += -O3
-    QMAKE_CFLAGS_OPTIMIZE_FULL += -Oz
-}
-
+QMAKE_LFLAGS_DEBUG  += -g
+QMAKE_LFLAGS_RELEASE  += -O2
 
 QMAKE_COMPILER         += emscripten