This commit adds a new option to the x86 assembler. If the
GOAMD64 environment variable is set to alignedjumps (the
default) and we're doing a 64 bit build, the assembler will
make sure that neither stand alone nor macro-fused jumps will
end on or cross 32 byte boundaries. To achieve this, functions
are aligned on 32 byte boundaries, rather than 16 bytes, and
jump instructions are padded to ensure that they do not
cross or end on 32 byte boundaries. Jumps are padded
by adding a NOP instruction of the appropriate length before
the jump.
The commit is likely to result in larger binary sizes when
GOAMD64=alignedjumps. On the binaries tested so far, an
increase of between 1.4% and 1.5% has been observed.