powerpc: Error on assembly warnings

We currently enable -Werror on the arch/powerpc subtree. However this
only catches C warnings. Assembly warnings are logged, but the make
invocation will still succeed. This can allow incorrect syntax such as

  ori r3, r4, r5

to be compiled without catching that the assembler is treating r5
as the immediate value 5.

To prevent this in assembly files and inline assembly, add the
-fatal-warnings option to assembler invocations.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Tested-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240326044420.577031-1-bgray@linux.ibm.com