commit 7a07de1efd4eb7cc11c33d3ad25cb2df70aa1ef1
author Pau Espin Pedrol <pespin@sysmocom.de> Wed Feb 19 16:31:57 2020 +0100
committer Pau Espin Pedrol <pespin@sysmocom.de> Wed Feb 19 18:08:20 2020 +0100
tree b18342affc780b9385cb8a9ef2f1e008c832241c
parent fd67262df85c367d44547131e129997b90aa8320

debug.h: Avoid printing pthread_t type

Using %lu for pthread_t was wrong on armv7l arch. Even worse, according
to pthread_self() man page, pthread_t cannot be assumed to be of a simple
type and hence printable:
"""
POSIX.1 allows an implementation wide freedom in choosing the type used
to represent a thread ID; for example, representation using either an
arithmetic  type  or a structure is permitted.
"""

Let's use gettid() instead. According to glibc documentation:
"""
The pid_t data type is a signed integer type which is capable of
representing a process ID. In the GNU C Library, this is an int.
"""
It may not be the same on other libc's though, so let's better cast to a
long int just in case.

Accordign to gettid() man, the libc function was only added recently
during glibc 2.30, however the system call has been around for quite
some time (linux 2.4.11). Let's accomodate use udner non-glibc or older
versions of it by having a direct syscall fallback.

Change-Id: I40265fd4c62e550014ba3ff3335ca053c5bc01f2