Support RISC-V cross compilation

[nhynes]

This allows TVM to build using a RISC-V GCC toolchain (https://github.com/rv8-io/musl-riscv-toolchain)

[hcho3]

Is using CMake an option for you?

[nhynes]

I'm using cmake for building the TVM runtime, but plain ol' make for the C++ runtime pack, as per the guide. I'd have to override the defines in either case. This just hides multiple -Ds behind one -D.

[hcho3]

@nhynes If you are using CMake all the way, CMake is able to detect endianness as well as the presence of backtrace libs:

dmlc-core/CMakeLists.txt

Lines 83 to 101 in 3ffea86

	# Check existence of backtrace(3)
	find_package(Backtrace)
	if(Backtrace_FOUND)
	set(DMLC_EXECINFO_H_PRESENT 1)
	set(DMLC_EXECINFO_H ${Backtrace_HEADER})
	include_directories(${Backtrace_INCLUDE_DIRS})
	list(APPEND dmlccore_LINKER_LIBS ${Backtrace_LIBRARIES})
	else()
	set(DMLC_EXECINFO_H_PRESENT 0)
	endif()
	# Check endianness
	include(TestBigEndian)
	test_big_endian(BIG_ENDIAN)
	if(BIG_ENDIAN)
	set(DMLC_CMAKE_LITTLE_ENDIAN 0)
	else()
	set(DMLC_CMAKE_LITTLE_ENDIAN 1)
	endif()

The point is moot, however, since you said you have to use plain Make. I will approve this pull request.