fix FLT_ROUNDS to reflect the current rounding mode

Implemented as a wrapper around fegetround introducing a new function
to the ABI: __flt_rounds. (fegetround cannot be used directly from float.h)

fix the nominal type of LDBL_* limits on archs with ld64

previously these macros wrongly had type double rather than long
double. I see no way an application could detect the error in C99, but
C11's _Generic can trivially detect it.

at the same time, even though these archs do not have excess
precision, the number of decimal places used to represent these
constants has been increased to 21 to be consistent with the decimal
representations used for the DBL_* macros.

add FLT_TRUE_MIN, etc. macros from C11

there was some question as to how many decimal places to use, since
one decimal place is always sufficient to identify the smallest
denormal uniquely. for now, I'm following the example in the C
standard which is consistent with the other min/max macros we already
had in place.

microblaze port

based on initial work by rdp, with heavy modifications. some features
including threads are untested because qemu app-level emulation seems
to be broken and I do not have a proper system image for testing.