Fix spillover of host-specific build flags into the shared part of DYNAMIC_ARCH builds with gmake

for #3139

Makefile.x86_64

@@ -8,6 +8,20 @@ endif
 endif
 endif
 
+
+ifndef DYNAMIC_ARCH
+define ADD_CPUFLAGS
+1
+endef
+else
+ifdef TARGET_CORE
+define ADD_CPUFLAGS
+1
+endef
+endif
+endif
+
+ifdef ADD_CPUFLAGS
 ifdef HAVE_SSE3
 CCOMMON_OPT += -msse3
 ifneq ($(F_COMPILER), NAG)
@@ -44,7 +58,6 @@ endif
 endif
 
 ifeq ($(CORE), SKYLAKEX)
-ifndef DYNAMIC_ARCH
 ifndef NO_AVX512
 CCOMMON_OPT += -march=skylake-avx512
 ifneq ($(F_COMPILER), NAG)
@@ -62,10 +75,8 @@ endif
 endif
 endif
 endif
-endif
 
 ifeq ($(CORE), COOPERLAKE)
-ifndef DYNAMIC_ARCH
 ifndef NO_AVX512
 ifeq ($(C_COMPILER), GCC)
 # cooperlake support was added in 10.1
@@ -88,7 +99,6 @@ endif
 endif
 endif
 endif
-endif
 
 ifdef HAVE_AVX2
 ifndef NO_AVX2
@@ -120,6 +130,7 @@ endif
 endif
 endif
 
+endif
 
 
 ifeq ($(OSNAME), Interix)