Provide a proper sqrtf declaration in math.h .
While we could have also get a functional header declaration using
__asm__ to make references use the sqrt symbol, it is preferable to
provide an actual sqrtf symbol in the library, so that code that
expects to be able to use sqrtf directly - like
gcc/gcc/testsuite/gcc.dg/pr41963.c - will work.
asm attribute
        * include/math.h (sqrtf): Don't #define, declare.
        * libm/fplib/asmdef.h (ALIAS_ENTRY): Define.
        * libm/fplib/sqrt.S (sqrtf): New ALIAS_ENTRY.