Implement real literal extension for Ada

Sometimes it is convenient to be able to specify the exact bits of a
floating-point literal.  For example, you may want to set a
floating-point register to a denormalized value, or to a particular
NaN.

In C, you can do this by combining the "{}" cast with an array
literal, like:

    (gdb) p {double}{0x576488BDD2AE9FFE}
    $1 = 9.8765449999999996e+112

This patch adds a somewhat similar idea to Ada.  It extends the lexer
to allow "l" and "f" suffixes in a based literal.  The "f" indicates a
floating-point literal, and the "l"s control the size of the
floating-point type.

Note that this differs from Ada's based real literals.  I believe
those can also be used to control the bits of a floating-point value,
but they are a bit more cumbersome to use (simplest is binary but
that's also very lengthy).  Also, these aren't implemented in GDB.

I chose not to allow this extension to work with based integer
literals with exponents.  That didn't seem very useful.