[cross] support arm-linux-gnueabihf cross compiling with netcore (#18714)

* [cross] support arm-linux-gnueabihf cross compiling with netcore

Some notes on usage:

```console
$ sudo apt install g{++,cc}-gnu-linux-gnueabihf python3-pip

$ ./autogen.sh --disable-boehm --target=arm-linux-gnueabihf \
    --with-cross-offsets=arm-linux-gnueabihf-cross-offsets.h \
    --with-core=only \
    CFLAGS="-O0 -ggdb3 -fno-omit-frame-pointer" \
    CXXFLAGS="-O0 -ggdb3 -fno-omit-frame-pointer"

$ make -C tools/offsets-tool-py/

$ python3 tools/offsets-tool-py/offsets-tool.py \
    --targetdir=/home/lewurm/work/mono \
    --abi=arm-linux-gnueabihf \
    --monodir=/home/lewurm/work/mono \
    --outfile=arm-linux-gnueabihf-cross-offsets.h \
    --libclang=/usr/lib/x86_64-linux-gnu/libclang-7.so.1 \
    --sysroot=/usr/arm-linux-gnueabihf \
    --netcore
```

* better include path guessing

1 files changed, 33 insertions, 0 deletions

diff --git a/tools/offsets-tool-py/offsets-tool.py b/tools/offsets-tool-py/offsets-tool.py
index f568a132b7e..d44e436e229 100644
--- a/tools/offsets-tool-py/offsets-tool.py
+++ b/tools/offsets-tool-py/offsets-tool.py
@@ -9,6 +9,7 @@ import clang.cindex
 
 IOS_DEFINES = ["HOST_DARWIN", "TARGET_MACH", "MONO_CROSS_COMPILE", "USE_MONO_CTX", "_XOPEN_SOURCE"]
 ANDROID_DEFINES = ["HOST_ANDROID", "MONO_CROSS_COMPILE", "USE_MONO_CTX", "BIONIC_IOCTL_NO_SIGNEDNESS_OVERLOAD"]
+LINUX_DEFINES = ["HOST_LINUX", "MONO_CROSS_COMPILE", "USE_MONO_CTX"]
 
 class Target:
 	def __init__(self, arch, platform, others):
@@ -61,6 +62,8 @@ class OffsetsTool:
 		parser.add_argument ('--targetdir', dest='target_path', help='path to mono tree configured for target', required=True)
 		parser.add_argument ('--abi=', dest='abi', help='ABI triple to generate', required=True)
 		parser.add_argument ('--sysroot=', dest='sysroot', help='path to sysroot headers of target')
+		parser.add_argument ('--include-prefix=', dest='include_prefix', help='prefix path to include directory of target')
+		parser.add_argument ('--netcore', dest='netcore', help='target runs with netcore', action='store_true')
 		args = parser.parse_args ()
 
 		if not args.libclang or not os.path.isfile (args.libclang):
@@ -84,6 +87,33 @@ class OffsetsTool:
 			self.target = Target ("TARGET_WASM", None, [])
 			self.target_args += ["-target", args.abi]
 
+		# Linux
+		elif "arm-linux-gnueabihf" == args.abi:
+			self.target = Target ("TARGET_ARM", None, ["ARM_FPU_VFP", "HAVE_ARMV5", "HAVE_ARMV6", "HAVE_ARMV7"] + LINUX_DEFINES)
+			self.target_args += ["--target=arm---gnueabihf"]
+			self.target_args += ["-I", args.sysroot + "/include"]
+
+			if args.include_prefix:
+				if not os.path.isdir (args.include_prefix):
+					print ("provided path via --include-prefix (\"" + args.include_prefix + "\") doesn't exist.", file=sys.stderr)
+					sys.exit (1)
+				self.target_args += ["-I", args.include_prefix + "/include"]
+				self.target_args += ["-I", args.include_prefix + "/include-fixed"]
+			else:
+				found = False
+				for i in range (11, 5, -1):
+					prefix = "/usr/lib/gcc-cross/" + args.abi +  "/" + str (i)
+					if not os.path.isdir (prefix):
+						continue
+					found = True
+					self.target_args += ["-I", prefix + "/include"]
+					self.target_args += ["-I", prefix + "/include-fixed"]
+					break
+
+				if not found:
+					print ("could not find a valid include path for target, provide one via --include-prefix=<path>.", file=sys.stderr)
+					sys.exit (1)
+
 		# iOS
 		elif "arm-apple-darwin10" == args.abi:
 			require_sysroot (args)
@@ -138,6 +168,9 @@ class OffsetsTool:
 			print ("ABI '" + args.abi + "' is not supported.", file=sys.stderr)
 			sys.exit (1)
 
+		if args.netcore:
+			self.target_args += ["-DENABLE_NETCORE"]
+
 		self.args = args
 
 	#