Don't hard-code GCC in Makefile

This uses the `CC` variable (which defaults to `cc`) so that compilation
works when using Clang. This also allows the compiler to be overridden
via `CC=foo make`.

Makefile

@@ -278,7 +278,7 @@ osim: ocaml_emulator/riscv_ocaml_sim_$(ARCH)
 rvfi: c_emulator/riscv_rvfi_$(ARCH)
 
 c_emulator/riscv_sim_$(ARCH): generated_definitions/c/riscv_model_$(ARCH).c $(C_INCS) $(C_SRCS) $(SOFTFLOAT_LIBS) Makefile
- gcc -g $(C_WARNINGS) $(C_FLAGS) $< $(C_SRCS) $(SAIL_LIB_DIR)/*.c $(C_LIBS) -o $@
+ $(CC) -g $(C_WARNINGS) $(C_FLAGS) $< $(C_SRCS) $(SAIL_LIB_DIR)/*.c $(C_LIBS) -o $@
 
 # Note: We have to add -c_preserve since the functions might be optimized out otherwise
 rvfi_preserve_fns=-c_preserve rvfi_set_instr_packet \
@@ -304,7 +304,7 @@ generated_definitions/c/riscv_rvfi_model_$(ARCH).c: $(SAIL_RVFI_SRCS) model/main
  mv $@.new $@
 
 c_emulator/riscv_rvfi_$(ARCH): generated_definitions/c/riscv_rvfi_model_$(ARCH).c $(C_INCS) $(C_SRCS) $(SOFTFLOAT_LIBS) Makefile
- gcc -g $(C_WARNINGS) $(C_FLAGS) $< -DRVFI_DII $(C_SRCS) $(SAIL_LIB_DIR)/*.c $(C_LIBS) -o $@
+ $(CC) -g $(C_WARNINGS) $(C_FLAGS) $< -DRVFI_DII $(C_SRCS) $(SAIL_LIB_DIR)/*.c $(C_LIBS) -o $@
 
 latex: $(SAIL_SRCS) Makefile
  mkdir -p generated_definitions/latex

c_emulator/SoftFloat-3e/build/Linux-RISCV-GCC/Makefile

@@ -44,7 +44,7 @@ SOFTFLOAT_OPTS ?= \
 DELETE = rm -f
 C_INCLUDES = -I. -I$(SOURCE_DIR)/$(SPECIALIZE_TYPE) -I$(SOURCE_DIR)/include
 COMPILE_C = \
- gcc -c -Werror-implicit-function-declaration \
+ $(CC) -c -Werror-implicit-function-declaration \
  $(SOFTFLOAT_OPTS) $(C_INCLUDES) -O2 -o $@
 MAKELIB = ar crs $@