libmach: accept OS X binary generated by external linker

Fixes cpu subtype check when using external linker which sets the CPU_SUBTYPE_LIB64 bit (1<<31).
Fixes issue 6197.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/13248046

Committer: Brad Fitzpatrick <bradfitz@golang.org>
cmd/6l, cmd/8l: fix MOVL MOVQ optab

The entry for LEAL/LEAQ in these optabs was listed as having
two data bytes in the y array. In fact they had and expect no data
bytes. However, the general loop expects to be able to look at at
least one data byte, to make sure it is not 0x0f. So give them each
a single data byte set to 0 (not 0x0f).

Since the MOV instructions have the largest optab cases, this
requires growing the size of the data array.

Clang found this bug because the general o->op[z] == 0x0f
test was using z == 22, which was out of bounds.

In practice the next byte in memory was probably not 0x0f
so it wasn't truly broken. But might as well be clean.

Update issue 5764

R=ken2
CC=golang-dev
https://codereview.appspot.com/13241050
build: more "undefined behavior" fixes

Fixes issue 5764.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13441051