[build] Defining EXE is not required

Some platforms don't have direct executable formats. For instance, on
Android, there *is* an EXE format for a given ARCH, but .so does not
apply to ARCH-less builds.

And having default rules for %.so on ARCH-less Android would get in
the way when copying the shared obejcts for the APK.

build/config.mak

@@ -16,9 +16,6 @@ ESCHER_LOG_EVENTS_BINARY ?= 0
 ifndef USE_LIBA
   $(error platform.mak should define USE_LIBA)
 endif
-ifndef EXE
-  $(error platform.mak should define EXE, the extension for executables)
-endif
 include build/toolchain.$(TOOLCHAIN).mak
 
 SFLAGS += -DDEBUG=$(DEBUG)

build/platform.simulator.android.mak

@@ -1,10 +1,10 @@
 TOOLCHAIN = android
-EXE = so
 
 EPSILON_TELEMETRY ?= 1
 
 ARCHS = armeabi-v7a arm64-v8a x86 x86_64
 
 ifdef ARCH
+EXE = so
 BUILD_DIR := $(BUILD_DIR)/$(ARCH)
 endif

build/rules.mk

@@ -43,6 +43,7 @@ $(eval $(call rule_for, \
   $$(CPP) -P $$< $$@ \
 ))
 
+ifdef EXE
 ifeq ($(OS),Windows_NT)
 # Work around command-line length limit
 # On Msys2 the max command line is 32 000 characters. Our standard LD command
@@ -59,6 +60,7 @@ $(eval $(call rule_for, \
   $$(LD) $$^ $$(LDFLAGS) -o $$@ \
 ))
 endif
+endif
 
 $(eval $(call rule_for, \
   WINDRES, %.o, %.rc, \