native: follow BUILD_* definitions for OBJCOPY, OBJDUMP and READELF

native: follow BUILD_* definitions for OBJCOPY, OBJDUMP and READELF

Set the host OBJCOPY, OBJDUMP, and READELF variables to be derived from
their corresponding BUILD_* definitions. This makes the native class
match the build-gcc.inc file 1 to 1, as these were the only missing.
Currently these variables get their definitions from gcc.inc, which uses
HOST_PREFIX, and that works because the native class sets HOST_PREFIX to
BUILD_PREFIX, but this doesn't seem correct.

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>