[arm] Don't use special treatment for PC

In an exception frame the PC register is extracted from the stack
just like other base registers, so there is no need for a special
treatment.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@st.com>
Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>