rpclib: slow floating point math and libm

In release-6 rpc/rpc-lib (libgfrpc) added the function
get_rightmost_set_bit() which calls log2(3), a call that takes
a floating point parameter and returns a floating point.

It's used thusly:
    right_most_unset_bit = get_rightmost_set_bit(...);

(So is it really the right-most unset bit, or the right-most set bit?)

It's unclear to me whether this is in the data path or not. If it is,
it's rather scary to think about integer-to-float and float-to-integer
conversions and slow calls to libm functions in the data path.

gcc and clang have __builtin_ctz() which returns the same result as
get_rightmost_set_bit(), and does it substantially faster. Approx
20M iterations of get_rightmost_set_bit() took ~33sec of wall clock
time on my devel machine, while 20M iterations of __builtin_ctz()
took < 9sec; get_rightmost_set_bit() is 3x slower than __builtin_ctz().

And as a side benefit, we can again eliminate the need to link libgfrpc
with libm.

Change-Id: If9e7e80874577c52223f8125b385fc930de20699
fixes: bz#1692957
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>

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