include/u.h: define _DEFAULT_SOURCE for new glibc

glibc devs have apparently decided _BSD_SOURCE will be
deprecated on Linux, and issue a preprocessor warning if
declaring _BSD_SOURCE without _DEFAULT_SOURCE.

https://sourceware.org/glibc/wiki/Release/2.20

Fixes issue 8397.

LGTM=iant
R=dave, gobot, iant
CC=golang-codereviews
https://codereview.appspot.com/112530043

Committer: Ian Lance Taylor <iant@golang.org>
runtime: convert complex128div to go.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/116320043
cmd/5l, cmd/6l, cmd/8l: remove mkenam.

Unused. cmd/dist will generate enams as liblink/anames[568].c.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/119940043
cmd/5l: remove unused noop.c

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/116330043
cmd/5g: remove dead code.

LGTM=bradfitz, dave
R=rsc, bradfitz, dave
CC=golang-codereviews
https://codereview.appspot.com/115070043
cmd/ld: remove duplicated nuxiinit() call.

It is already called by linknew().

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/120760043
liblink: warn about TLS base MRC instruction that does not write into R0.

While we're here, make it lookup the tlsfallback symbol only once.

LGTM=crawshaw
R=golang-codereviews, crawshaw, dave
CC=golang-codereviews
https://codereview.appspot.com/107430044