riscv-binutils-2.28 now has GDB support again.

This reverts commit 2490d7ca.

gdb-sim accounted for nearly all test failures, so don't use it for now.

For some reason, a Newlib multilib build includes a "default" library
in addition to the multilibs ones.  This results in confounding error
messages when an invalid multilib is requested on the GCC command line.
Now, it reports a more sensible "cannot find -lc".

- -march controls which instructions may be emitted, and does not
affect ABI.
- -mno-float is equivalent to passing neither F for D to -march;
-msingle-float is equivalent to passing F but not D to -march;
and -mdouble-float is equivalent to passing F and D to -march.
- -mfloat-abi controls which calling convention is used: "soft" means
no args passed in registers; "single" means only single-precision
values are passed in regisers; "double" means single- and
double-precision values are passed in registers.
- -mfloat-abi defaults to "soft" if the D extension is not present,
or "double" if it is.
- GCC will issue an error if -mfloat-abi requires an extension not
provided by -march, but vice-versa is OK.

Closes #187 (I hope).

@kito-cheng can you test this and let me know what I need to fix?

Soft-float calling convention is default, but an alternate hard-float
calling convention (with doubles passed as in soft-float, but floats
passed as in hard-float) is supported.

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