Use the gold linker only for target builds

Availability of the gold linker is only tested with the
cross-compiler, not the host compiler, so Qt shouldn't assume it's
available when doing host builds.

This fixes build failures occuring when cross-compiling Qt5 with a
gold capable cross-compiler, on a host that has a too old compiler to
support gold.

Bug reported upstream at https://bugreports.qt.io/browse/QTBUG-46125.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/mkspecs/features/default_post.prf
===================================================================
--- a/mkspecs/features/default_post.prf
+++ b/mkspecs/features/default_post.prf
@@ -62,7 +62,7 @@
     QMAKE_LIBFLAGS += $$QMAKE_LIBFLAGS_RELEASE
 }
 
-use_gold_linker: QMAKE_LFLAGS += $$QMAKE_LFLAGS_USE_GOLD
+!host_build: use_gold_linker: QMAKE_LFLAGS += $$QMAKE_LFLAGS_USE_GOLD
 
 dll:win32: QMAKE_LFLAGS += $$QMAKE_LFLAGS_DLL
 static:mac: QMAKE_LFLAGS += $$QMAKE_LFLAGS_STATIC_LIB