libm: No divtc3.c or multc3.c in sparc64's compat sparc library.

compat/sparc64/sparc/bsd.sparc.mk doesn't define LIBC_MACHINE_CPU,
only LIBC_MACHINE_ARCH, so when the compat build gets to this
conditional, LIBC_MACHINE_CPU is just MACHINE_CPU, i.e., sparc64.

Since there's no `sparc64el' or `sparc64hf-el' that we need to
canonicalize by MACHINE_ARCH -> MACHINE_CPU, just use
LIBC_MACHINE_ARCH here.

(riastradh)