ld: preserve symbol sizes during data layout

ld: preserve symbol sizes during data layout

Fixes the broken linux/amd64 build.
The symbol table, itself a symbol, was having
its size rounded up to the nearest word boundary.
If the rounding add >7 zero bytes then it confused
the debug/gosym symbol table parser.  So you've
got a 1/8 chance to hit the bug on an amd64 system.

Just started in the recent change because I fixed
the rounding to round to word boundary instead
of to 4-byte boundary.

R=r
CC=golang-dev
https://golang.org/cl/4241056