Stop using STATIC_CFLAGS.

This was added in r293648 to pass -mlong-calls for crt1.o and gcrt1.o.
The use of -mlong-calls was removed in r358851 for LLVM 10.0, leaving
STATIC_CFLAGS empty.

Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25305