target-arm: use tcg_global_mem_new_i32 to allocate registers

target-arm: use tcg_global_mem_new_i32 to allocate registers

Currently each read/write of ARM register involves a LD/ST TCG operation. This
patch uses TCG memory-backed registers to represent the ARM register set. With
memory-backed registers the LD/ST operations are transparently generated by TCG
and host registers could be used to optimize the generated code.

Signed-off-by: Filip Navara <filip.navara@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>