Check for F16c and rdrand support always passes and compiles into the library support for these

I'm running into the following issue after letting Gentoo's ebuild build 5.9.2.  I get this message when launching newly compiled Qt applications:
Incompatible processor. This Qt build requires the following features:
f16c rdrand
Aborted. Incompatible processor: missing feature 0x60000000 - f16c.
Aborted
Tracking this down seems to originate from the fact that while the f16c test is sufficient for compiler support, it is not sufficient for whether or not this instruction is supported.  If one runs the f16c test code, it passes with return code 0, as it's compiled with -O2.  Because the optimizer optimizes out the nonconsequential half precision operations, the build system incorrectly assumes f16c and rdrand instructions are supported.

Please seem my bug on the Gentoo bugzilla for more info:

https://bugs.gentoo.org/634458