AArch64: Use ZR for integer zero immediate volatile stores

In case non-volatile zero byte, char, int, long or pointer is stored C2 generates code like

str xzr, [x21, #24]

There are appropriate rules and encodings for that in aarch64.ad like
storeimmB0() and aarch64_enc_strb0().

For volatiles there are no such rules for 0 while all the rest is similar. Current code is like:

mov x13, xzr
stlr x13, [x11]

One register can be saved and 1 instruction instead of 2 can be used:

stlr xzr [x11]