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.

initial version of mips (o32) port, based on work by Richard Pennington (rdp)

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.