Overhaul floating-point command-line arguments

- -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?

1 files changed, 0 insertions, 0 deletions

diff --git a/riscv-glibc b/riscv-glibc
-Subproject 8b53d22b5670a4780e1e2a5ee4198c1cef37204
+Subproject 1a4cb9a9aba93d6a279a6d8c42036e7e97bff89