Adds many more cases of ldr{b,h}/str{b,h} instructions to the ARM
integerated assembler.

Also refactored decodeAddress() to deal with the fact that
immediate/register arguments for LDR/LDRB/STR/STRB differ for
LDRH/LDRD/STRH/STRD, by passing in the layout wanted.

Note: LDRD/STRD instructions are currently not used by Subzero.

BUG=https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1486263002 .