read_frame_register_value and big endian arches

read_frame_register_value and big endian arches

The read_frame_register_value function as it was implemented introduced
a regression on big-endian targets. The problem appears when trying to
get the value of an entity stored inside a register, and when the size
of the entity is smaller than the size of the register. In that case,
we were always reading the first N bytes of the register, which is wrong
for big-endian architectures, where we need to read the last N bytes of
the register.

gdb/ChangeLog:

        * findvar.c (read_frame_register_value): Read correct bytes from
        register on big-endian architectures.

gdb/testsuite/ChangeLog:

        * gdb.ada/small_reg_param: New testcase.