ports/unix/Makefile: Don't assume Linux by default.

Motivation: allow to override this with "make UNAME_S=wasi"

Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>

ports/unix/Makefile

@@ -103,7 +103,7 @@ CC = clang
 endif
 # Use clang syntax for map file
 LDFLAGS_ARCH = -Wl,-map,$@.map -Wl,-dead_strip
-else
+else ifeq ($(UNAME_S),Linux)
 # Use gcc syntax for map file
 LDFLAGS_ARCH = -Wl,-Map=$@.map,--cref -Wl,--gc-sections
 endif