core/sys/unix: Add syscalls_linux.odin

Linux is in the unfortunate situation where the system call number is
architecture specific.  This consolidates the system call number
definitions in a single location, adds some wrappers, and hopefully
fixes the existing non-portable invocations of the syscall intrinsic.

core/os/os_linux.odin

@@ -8,6 +8,7 @@ import "core:strings"
 import "core:c"
 import "core:strconv"
 import "core:intrinsics"
+import "core:sys/unix"
 
 Handle    :: distinct i32
 File_Time :: distinct u64
@@ -265,8 +266,6 @@ X_OK :: 1 // Test for execute permission
 W_OK :: 2 // Test for write permission
 R_OK :: 4 // Test for read permission
 
-SYS_GETTID :: 186
-
 foreign libc {
 	@(link_name="__errno_location") __errno_location    :: proc() -> ^int ---
 
@@ -594,7 +593,7 @@ exit :: proc "contextless" (code: int) -> ! {
 }
 
 current_thread_id :: proc "contextless" () -> int {
-	return cast(int)intrinsics.syscall(SYS_GETTID)
+	return unix.sys_gettid()
 }
 
 dlopen :: proc(filename: string, flags: int) -> rawptr {