tools/mpy_ld.py: Add new mpy_ld.py tool and associated build files.

This commit adds a new tool called mpy_ld.py which is essentially a linker
that builds .mpy files directly from .o files.  A new header file
(dynruntime.h) and makefile fragment (dynruntime.mk) are also included
which allow building .mpy files from C source code.  Such .mpy files can
then be dynamically imported as though they were a normal Python module,
even though they are implemented in C.

Converting .o files directly (rather than pre-linked .elf files) allows the
resulting .mpy to be more efficient because it has more control over the
relocations; for example it can skip PLT indirection.  Doing it this way
also allows supporting more architectures, such as Xtensa which has
specific needs for position-independent code and the GOT.

The tool supports targets of x86, x86-64, ARM Thumb and Xtensa (windowed
and non-windowed).  BSS, text and rodata sections are supported, with
relocations to all internal sections and symbols, as well as relocations to
some external symbols (defined by dynruntime.h), and linking of qstrs.

py/dynruntime.mk

@@ -0,0 +1,134 @@
+# Makefile fragment for generating native .mpy files from C source
+# MPY_DIR must be set to the top of the MicroPython source tree
+
+BUILD ?= build
+
+ECHO = @echo
+RM = /bin/rm
+MKDIR = /bin/mkdir
+PYTHON = python3
+MPY_CROSS = $(MPY_DIR)/mpy-cross/mpy-cross
+MPY_TOOL = $(PYTHON) $(MPY_DIR)/tools/mpy-tool.py
+MPY_LD = $(PYTHON) $(MPY_DIR)/tools/mpy_ld.py
+
+Q = @
+ifeq ("$(origin V)", "command line")
+ifeq ($(V),1)
+Q =
+MPY_LD += '-vvv'
+endif
+endif
+
+ARCH_UPPER = $(shell echo $(ARCH) | tr '[:lower:]' '[:upper:]')
+CONFIG_H = $(BUILD)/$(MOD).config.h
+
+CFLAGS += -I. -I$(MPY_DIR)
+CFLAGS += -std=c99
+CFLAGS += -Os
+CFLAGS += -Wall -Werror -DNDEBUG
+CFLAGS += -DNO_QSTR
+CFLAGS += -DMP_CONFIGFILE='<$(CONFIG_H)>'
+CFLAGS += -fpic -fno-common
+CFLAGS += -U _FORTIFY_SOURCE # prevent use of __*_chk libc functions
+#CFLAGS += -fdata-sections -ffunction-sections
+
+MPY_CROSS_FLAGS += -march=$(ARCH)
+
+SRC_O += $(addprefix $(BUILD)/, $(patsubst %.c,%.o,$(filter %.c,$(SRC))))
+SRC_MPY += $(addprefix $(BUILD)/, $(patsubst %.py,%.mpy,$(filter %.py,$(SRC))))
+
+################################################################################
+# Architecture configuration
+
+ifeq ($(ARCH),x86)
+
+# x86
+CROSS =
+CFLAGS += -m32 -fno-stack-protector
+MPY_CROSS_FLAGS += -mcache-lookup-bc
+
+else ifeq ($(ARCH),x64)
+
+# x64
+CROSS =
+CFLAGS += -fno-stack-protector
+MPY_CROSS_FLAGS += -mcache-lookup-bc
+
+else ifeq ($(ARCH),armv7m)
+
+# thumb
+CROSS = arm-none-eabi-
+CFLAGS += -mthumb -mcpu=cortex-m3
+
+else ifeq ($(ARCH),armv7emsp)
+
+# thumb
+CROSS = arm-none-eabi-
+CFLAGS += -mthumb -mcpu=cortex-m4
+CFLAGS += -mfpu=fpv4-sp-d16 -mfloat-abi=hard
+
+else ifeq ($(ARCH),armv7emdp)
+
+# thumb
+CROSS = arm-none-eabi-
+CFLAGS += -mthumb -mcpu=cortex-m7
+CFLAGS += -mfpu=fpv5-d16 -mfloat-abi=hard
+
+else ifeq ($(ARCH),xtensa)
+
+# xtensa
+CROSS = xtensa-lx106-elf-
+CFLAGS += -mforce-l32
+
+else ifeq ($(ARCH),xtensawin)
+
+# xtensawin
+CROSS = xtensa-esp32-elf-
+CFLAGS +=
+
+else
+$(error architecture '$(ARCH)' not supported)
+endif
+
+CFLAGS += $(CFLAGS_EXTRA)
+
+################################################################################
+# Build rules
+
+.PHONY: all clean
+
+all: $(MOD).mpy
+
+clean:
+	$(RM) -rf $(BUILD) $(CLEAN_EXTRA)
+
+# Create build destination directories first
+BUILD_DIRS = $(sort $(dir $(CONFIG_H) $(SRC_O) $(SRC_MPY)))
+$(CONFIG_H) $(SRC_O) $(SRC_MPY): | $(BUILD_DIRS)
+$(BUILD_DIRS):
+	$(Q)$(MKDIR) -p $@
+
+# Preprocess all source files to generate $(CONFIG_H)
+$(CONFIG_H): $(SRC)
+	$(ECHO) "GEN $@"
+	$(Q)$(MPY_LD) --arch $(ARCH) --preprocess -o $@ $^
+
+# Build .o from .c source files
+$(BUILD)/%.o: %.c $(CONFIG_H) Makefile
+	$(ECHO) "CC $<"
+	$(Q)$(CROSS)gcc $(CFLAGS) -o $@ -c $<
+
+# Build .mpy from .py source files
+$(BUILD)/%.mpy: %.py
+	$(ECHO) "MPY $<"
+	$(Q)$(MPY_CROSS) $(MPY_CROSS_FLAGS) -o $@ $<
+
+# Build native .mpy from object files
+$(BUILD)/$(MOD).native.mpy: $(SRC_O)
+	$(ECHO) "LINK $<"
+	$(Q)$(MPY_LD) --arch $(ARCH) --qstrs $(CONFIG_H) -o $@ $^
+
+# Build final .mpy from all intermediate .mpy files
+$(MOD).mpy: $(BUILD)/$(MOD).native.mpy $(SRC_MPY)
+	$(ECHO) "GEN $@"
+	$(Q)$(MPY_TOOL) --merge -o $@ $^