cpu/esp32: cleanup of C++ hacks

Since former ESP32 toolchain versions used POSIX threads, module `pthread` was required. The built-in `cxa_ctor_guards` had to be replaced since they used the `pthread_once` function for singleton objects initialization where the parameter `once` was of incompatible type with that provided by RIOT's `pthread` module. The current ESP32 toolchain version no longer uses POSIX threads. The dependency on module `pthread` as well as according C++ hacks can be removed.

cpu/esp32/Makefile

@@ -6,10 +6,6 @@ DIRS += $(RIOTCPU)/esp_common
 DIRS += periph
 DIRS += vendor
 
-ifneq (, $(filter esp_cxx, $(USEMODULE)))
-    DIRS += cxx
-endif
-
 ifneq (, $(filter esp_eth, $(USEMODULE)))
     DIRS += esp-eth
 endif

cpu/esp32/Makefile.dep

@@ -8,11 +8,6 @@ USEMODULE += esp_idf_soc
 USEMODULE += newlib_nano
 USEMODULE += pm_layered
 
-ifneq (,$(filter cpp,$(FEATURES_USED)))
-  USEMODULE += pthread
-  USEMODULE += esp_cxx
-endif
-
 ifneq (,$(filter esp_eth,$(USEMODULE)))
   USEMODULE += esp_freertos
   USEMODULE += esp_idf_eth
@@ -41,7 +36,6 @@ endif
 ifneq (,$(filter esp_idf_nvs_flash,$(USEMODULE)))
   # add additional modules required by esp_idf_nvs_flash
   USEMODULE += mtd
-  USEMODULE += pthread
 endif
 
 ifneq (,$(filter periph_rtc,$(USEMODULE)))

cpu/esp32/Makefile.include

@@ -71,7 +71,6 @@ ifneq (,$(filter esp_now,$(USEMODULE)))
 endif
 
 ifneq (,$(filter cpp,$(FEATURES_USED)))
-  UNDEF += $(BINDIR)/esp_cxx/cxa_guard.o
   BASELIBS += -lstdc++
 endif