Explicitly say upper bit in GPR is undefined when passing floating point value in hard fp CC.

Fix #267.

Co-authored-by: Andrew Waterman <[email protected]>

Author: kito-cheng

riscv-cc.adoc

@@ -234,10 +234,12 @@ floating-point reals.
 
 A struct containing one floating-point real and one integer (or bitfield), in
 either order, is passed in a floating-point register and an integer register,
-without extending the integer to XLEN bits, provided the floating-point real
-is no more than ABI_FLEN bits wide and the integer is no more than XLEN bits wide,
-and at least one floating-point argument register and at least one integer
-argument register is available. Otherwise, it is passed according to the
+provided the floating-point real is no more than ABI_FLEN bits wide and the
+integer is no more than XLEN bits wide, and at least one floating-point
+argument register and at least one integer argument register is available.
+If the struct is passed in this manner, and the integer is narrower than XLEN
+bits, the remaining bits are unspecified.
+If the struct is not passed in this manner, then it is passed according to the
 integer calling convention.
 
 Unions are never flattened and are always passed according to the integer