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
net/http: make ServeContent support dates in If-Range headers

Fixes Issue 8367

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/116300044