Force export symbols on all x86 variants in libc.

For silvermont, the __popcountsi2 symbol does not get exported by libc.
But for atom, this symbol is exported. Since we already exported this symbol
for previous releases, it's better to just follow through and force
the export, but only for 32 bit. x86 64 bit will not export this symbol.

Bug: 17681440

(cherry picked from commit d11eac34)

Change-Id: I93704c721d98d569922f606f214069bda24872ba