devel/gdb: Fix kernel backend registrations.

Per-architecture kernel backends are registered via GDB initialization
routines. The GDB build uses a regular expression to identify the
initialization routines invoked during startup. This regular
expression requires a single space between the function name and it's
empty argument list. However, the initializers for aarch64, arm,
mips, and riscv had two spaces before the empty argument list causing
those backends to not be registered and the corresponding architecture
kernels to not be recognized. Fix by removing the spurious spaces.

Reported by: kevans
Reviewed by: pizzamig (maintainer)
Differential Revision: https://reviews.freebsd.org/D33255