libc: Simplify soft-float on 32-bit arm

Simplify the tests for 32-bit arm soft float support. For the files
included only on arm, drop the test entirely. For others, test
MACHINE_CPUARCH against arm.

No functional change intended. File lists appear the same before / after
the change.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38582