Added x86_64 registers temporarily.

Just added the 64 bits register to get through compilation.
The entire sr_arm_regs directory will have to be remade to
fit the ARM architecture registers.
Enabling declarations of rtnhdr_off labaddr_off

Apparently used for code generation.
Added headers to sr_arm from sr_x86_64.
Added under of TRUTH_IN_REG for linux + arm.
Temporarily declare CALL_INST_SIZE.

This choice must be reviewed. It is unclear why when CALL_INST_SIZE
is used, it is under the conditional of (not) x86_64, but the symbol
is only declared on the sr_alpha headers.

We will get back here when we start generating code...
Compiled emit_code.c by making ARM look like 64.

Make several macros avalable to __arm__, that were intended for x86_64.