[merp] MONO_DEBUG=no-gdb-stacktrace shouldn't disable MERP (#2149)

[merp] MONO_DEBUG=no-gdb-stacktrace shouldn't disable MERP (#2149)

Only prevent `gdb` or `lldb` from being invoked.

MERP is controlled by separate mechanisms --- configure flags for crash
reporting and an explicit opt-in icall.

The existing code collects crashed process data in-process, but it writes the
data out (using mono_merp_invoke) in the forked child process.  This is a bit
surprising, but in the interest of not disturbing the code too much, this PR
doesn't change that.  We should revisit `dump_native_stacktrace` again in the
future and rationalize these decisions.

Addresses https://github.com/mono/mono/issues/18565

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>