powerpc/boot: Build wrapper for an appropriate CPU

Currently the boot wrapper lacks a -mcpu option, so it will be built for
the toolchain's default cpu. This is a problem if the toolchain defaults
to a cpu with newer instructions.

We could wire in TARGET_CPU but instead use the oldest supported option
so the wrapper runs anywhere.

The GCC documentation stays that -mcpu=powerpc64le will give us a
generic 64 bit powerpc machine:

 -mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure
 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit
 little endian PowerPC architecture machine types, with an appropriate,
 generic processor model assumed for scheduling purposes.

So do that for each of the three machines.

This bug was found when building the kernel with a toolchain that
defaulted to powre10, resulting in a pcrel enabled wrapper which fails
to link:

 arch/powerpc/boot/wrapper.a(crt0.o): in function `p_base':
 (.text+0x150): call to `platform_init' lacks nop, can't restore toc; (toc save/adjust stub)
 (.text+0x154): call to `start' lacks nop, can't restore toc; (toc save/adjust stub)
 powerpc64le-buildroot-linux-gnu-ld: final link failed: bad value

Even with tha bug worked around the resulting kernel would crash on a
power9 box:

 $ qemu-system-ppc64 -nographic -nodefaults -M powernv9 -kernel arch/powerpc/boot/zImage.epapr -serial mon:stdio
 [    7.069331356,5] INIT: Starting kernel at 0x20010020, fdt at 0x3068c628 25694 bytes
 [    7.130374661,3] ***********************************************
 [    7.131072886,3] Fatal Exception 0xe40 at 00000000200101e4    MSR 9000000000000001
 [    7.131290613,3] CFAR : 000000002001027c MSR  : 9000000000000001
 [    7.131433759,3] SRR0 : 0000000020010050 SRR1 : 9000000000000001
 [    7.131577775,3] HSRR0: 00000000200101e4 HSRR1: 9000000000000001
 [    7.131733687,3] DSISR: 00000000         DAR  : 0000000000000000
 [    7.131905162,3] LR   : 0000000020010280 CTR  : 0000000000000000
 [    7.132068356,3] CR   : 44002004         XER  : 00000000

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
BugLink: https://github.com/linuxppc/issues/issues/400
Link: https://lore.kernel.org/r/20220330112437.540214-1-joel@jms.id.au

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