cmd/ld: fix some 64-bit issues
A few places in the linker pushed 64-bit values through 32-bit holes,
including in relocation.
Clean them up, and check for a few other overflows as well.
Tests to follow.

R=dsymonds
CC=gobot, golang-dev
https://codereview.appspot.com/9032043
cmd/gc: fix some overflows in the compiler
Some 64-bit fields were run through 32-bit words, some counts were
not checked for overflow, and relocations must fit in 32 bits.
Tests to follow.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/9033043