RFC: Run libm tests separately for each function

[Florian Weimer]

* Joseph Myers:

> Any comments?  Especially regarding the use of iterators; there is
> existing precedent (in elf/Makefile) for using o-iterator.mk as a
> generic iterator with object-suffixes-left set to something other than
> a list of object suffixes, but maybe there should be a differently
> named iterator for such generic uses?

Does this patch depend on another change?  I get a build failure on
x86-64:

In file included from ../sysdeps/x86_64/fpu/test-double-libmvec-alias-mod.c:25:0,
                 from ../sysdeps/x86_64/fpu/test-double-libmvec-alias-avx512-mod.c:1:
../sysdeps/x86_64/fpu/test-libmvec-alias-mod.c: In function ‘init_arg’:
../sysdeps/x86_64/fpu/test-libmvec-alias-mod.c:33:3: error: implicit declaration of function ‘GLRO’ [-Werror=implicit-function-declaration]
   CHECK_ARCH_EXT;
   ^

My immediate concern is that o-iterator.mk appears to have quadratic
behavior in the number of elements.  I'd like to run some build-time
benchmarks to ensure that this has no excessive build-time impact.