[NetBSD] Use cortex-a8 as default CPU for ARMv7

This matches the platform default for GCC. It primarily matters when the
integrated assembler is not used as there is no default CPU defined for
ARMv7-A and GNU as is upset with -mcpu=generic.

GitOrigin-RevId: 2628e9146120507b2cf025f5c4ccc857cc3724c4
1 file changed