[libc] fix -Wshorten-64-to-32 for 32b arm mmap

[nickdesaulniers]

#77350 was an attempted fix that didn't work. Filing a TODO to track a quick workaround to get the build bot green, until we can fix this properly.

[nickdesaulniers]

/usr/include/arm-linux-gnueabihf/sys/mman.h on the 32b arm buildbot has:

#ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
#endif

and preprocessing libc/src/sys/mman/linux/mmap.cpp I see:

# 26 "/usr/include/arm-linux-gnueabihf/sys/mman.h" 2 3 4
typedef __off64_t off_t;

so that's where the 64b off_t is coming from.

[nickdesaulniers]

We have a libc/include/llvm-libc-types/off_t.h, should libc/src/sys/mman/mmap.h be explicitly including that rather than <sys/mman.h>?

[nickdesaulniers]

I see _LARGEFILE_SOURCE getting defined when building this TU, but...I only see that being added to the compile options in
llvm/cmake/modules/HandleLLVMOptions.cmake, and llvm/cmake/config-ix.cmake.

That's not polluting our build flags, is it?

Otherwise, perhaps we need to provide an implementation of the syscall that accepts a long long for that param.

[nickdesaulniers]

llvm-project/llvm/cmake/modules/HandleLLVMOptions.cmake

Lines 446 to 450 in 408dce8

	if( CMAKE_SIZEOF_VOID_P EQUAL 4 AND NOT LLVM_FORCE_SMALLFILE_FOR_ANDROID)
	# FIXME: It isn't handled in LLVM_BUILD_32_BITS.
	add_compile_definitions(_LARGEFILE_SOURCE)
	add_compile_definitions(_FILE_OFFSET_BITS=64)
	endif()

is leaking into our build rules, which is what's forcing off_t to be 64b on 32b targets. I suspect we may want to avoid that, but need to think more about the implications of disabling that.

I suspect it may be a question of (do we want to match the platform's ABI vs do our own ABI).

[nickdesaulniers]

FWICT, it looks like we're inheriting:

-- Found Define: _GNU_SOURCE
-- Found Define: _FILE_OFFSET_BITS=64
-- Found Define: _DEBUG
-- Found Define: _GLIBCXX_ASSERTIONS
-- Found Define: _LARGEFILE_SOURCE
-- Found Define: _FILE_OFFSET_BITS=64
-- Found Define: __STDC_CONSTANT_MACROS
-- Found Define: __STDC_FORMAT_MACROS
-- Found Define: __STDC_LIMIT_MACROS

preprocessor defines. I don't think we want any of those (at least for now).