[ARM] Refactor strlen.c #if nesting.

[ARM] Refactor strlen.c #if nesting.

This patch flattens the condition code selection used in strlen in an
attempt to make the guarding condition for each alternative
implementation clearer and to structure the logic in a manner that
makes it easier to maintain complementary logic between the
alternative 'C' and assembler implementations.

Tested by building newlib and comparing libc.a binaries before and
after for all permutations of:

  Architectures:
    armv4 armv4t armv5 armv5t armv5te armv6 armv6j armv6k
    armv6z armv6kz armv6t2 armv6-m armv6s-m armv7 armv7-a
    armv7ve armv7-r armv7-m armv7e-m armv8-a iwmmxt iwmmxt2

  ISAs:
    thumb arm

  Optimization Levels:
    Os O2

  Excluding:
    armv6s-m -mthumb
    armv6-m -mthumb

    armv6zk -mthumb
    armv6z -mthumb
    armv6k -mthumb
    armv6j -mthumb