gdb/arm: Fix prologue analysis to support vpush

While working on adding support for Non-secure/Secure modes unwinding,
I noticed that the prologue analysis lacked support for vpush, which
is used for instance in the CMSE stub routine.

This patch updates thumb_analyze_prologue accordingly, adding support
for vpush of D-registers.

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