PermissionsDarwin wiki fix

Hi,

Information is missing in https://sourceware.org/gdb/wiki/PermissionsDarwin

On macOS 10.15, when you create the certificate, the default alias is set
to your email address. This needs to be set to gdb-cert too. This is
implicit in the `security find-certificate -c gdb-cert` output but is not
specified.

Without that, when you get to the `codesign --entitlements
gdb-entitlement.xml -fs gdb-cert $(which gdb)` stage, you get:

error: The specified item could not be found in the keychain.

Can someone update the wiki?

Thanks
Galder