gnu: fftw: Fix configure flags for armhf, aarch64, and mips.

gnu: fftw: Fix configure flags for armhf, aarch64, and mips.

This is a followup to commit 65bb22796f854cbc3eae053a80b1d64365dad376.

* gnu/packages/algebra.scm (fftw)[arguments]: In the configure-flags, avoid
two-layer quasiquotation by changing the inner quasiquote to a normal quote.
Remove "--enable-armv7a-cntvct" on 32-bit ARM, and "--enable-armv8-cntvct-el0"
on 64-bit ARM, since these generate instructions that are not normally
available from user mode.  Remove "--enable-mips-zbus-timer" on MIPS, since
the needed hardware support may not be available.  Add a default case to the
conditional to support unrecognized targets.
(fftwf)[arguments]: Restore the "--enable-single" flag, which was present
prior to commit 65bb22796.