makesyscalls: Add a way to include per-ABI headers

When the string %%ABI_HEADERS%% is found in syscalls.master, replace
it with the contents of the abi_headers config variable.  This allows
an ABI-specific syscalls.conf to add lines like:

	#include <compat/freebsd32/freebsd32.h>

when working from a shared syscalls.master.

Reviewed by:	kevans

sys/tools/makesyscalls.lua

@@ -60,6 +60,7 @@ local config = {
 	abi_type_suffix = "",
 	abi_flags = "",
 	abi_flags_mask = 0,
+	abi_headers = "",
 	ptr_intptr_t_cast = "intptr_t",
 }
 
@@ -438,6 +439,16 @@ local pattern_table = {
 			write_line_pfile('systrace.*', line)
 		end,
 	},
+	{
+		dump_prevline = true,
+		pattern = "%%ABI_HEADERS%%",
+		process = function()
+			if config['abi_headers'] ~= "" then
+				line = config['abi_headers'] .. "\n"
+				write_line('sysinc', line)
+			end
+		end,
+	},
 	{
 		-- Buffer anything else
 		pattern = ".+",