gdb/arm: Introduce arm_cache_init

This patch is a preparation for the rest of the series and adds two
arm_cache_init helper functions. It updates every place that updates
cache->saved_regs to call the helper instead.

Signed-off-by: Torbjörn Svensson <torbjorn.svensson@st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@foss.st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@arm.com>