Read Ada main name from executable, not inferior

An upstream bug report points out this bug: if the user switches from
one Ada executable to another without "kill"ing the inferior, then the
"start" command will fail.

What happens here is that the Ada "main" name is found in a constant
string in the executable.  But, if the inferior is running, then the
process_stratum target reads from the inferior memory.

This patch fixes the problem by changing the main name code to set
trust-readonly-sections, causing the target stack to read from the
executable instead.

I looked briefly at changing GNAT to emit DW_AT_main_subprogram
instead, but this looks to be pretty involved.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=25811