toolchain/musl: update to version 1.1.20

toolchain/musl: update to version 1.1.20

This release introduces the ability to replace/interpose the allocator
(malloc) subject to certain restrictions, adds an experimental m68k
port, and makes notable improvements to stdio (application-provided
buffers), getaddrinfo (AI_ADDRCONFIG, support for IPv4-only kernel
configurations), the dynamic linker (safety against dlopen of
libraries using initial-exec TLS model, reclaiming unused memory on
FDPIC archs, better dladdr results), and handling of default thread
stack size (pthread_setattr_default_np now works more reliably).

Many bugs have been fixed, including potentially dangerous regressions
in iconv (only for new conversions to legacy encodings) and visibly
incorrect behavior in printf on non-x86 archs (%a format with
precision specifier), in getopt_long_only when short options are a
prefix for a long option, in complex arc-trig/hyperbolic functions, in
strftime and mktime (timezone-specific issues), and numerous
less-obvious places.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[altered commit msg a bit keeping it tight]
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(backported from 1801e60390706abc0ffe6376c5d9441ca10253c6)