Log Message:
gcc7: fix many netbsd targets.

add netbsd/arm EABI target. recognise dwarf2 unwinding in the common arm code.

switch convoluted stddef.h logic from relying on include guards having certain
names to only applying for netbsd, which should be the sole remaining user.
(necessary for netbsd/arm, which uses different include guards for ansi.h)

move linux/alpha code out of shared alpha+ELF header.

make all netbsd targets include netbsd-stdint.h.

Fixes PR pkg/52951.

Bump PKGREVISION. bump gcc7-libs PKGREVISION above this one.