bsp/motorola_powerpc: Remove obsolete GCC option

commit 20c89ab7c5091ee48535392cae2177aa1a1c43eb
Author: Segher Boessenkool <segher@kernel.crashing.org>
Date:   Fri Jan 12 21:50:52 2018 +0100

    rs6000: Remove -mstring

    -mstring is only enabled by default on 601, and with -Os on some
    configurations.  It is almost always slower (than not using it) and
    does not very often lead to smaller code.

    This patch disables it.  If a user uses -mstring he gets a warning
    (but not with -mno-string).  I left the target attribute in place, it
    just doesn't do anything anymore.

    The patch also deletes a whole bunch of code.  The 'N' and 'O' output
    modifiers are now unused, but now is not the time to delete them.

spec/build/bsps/powerpc/motorola_powerpc/abi.yml

@@ -12,21 +12,18 @@ default-by-variant:
 - value:
   - -mcpu=7400
   - -mmultiple
-  - -mstring
   - -mstrict-align
   variants:
   - powerpc/qemuprep-altivec
 - value:
   - -mcpu=powerpc
   - -mmultiple
-  - -mstring
   - -mstrict-align
   variants:
   - powerpc/qemuprep
 - value:
   - -mcpu=604
   - -mmultiple
-  - -mstring
   - -mstrict-align
   - -meabi
   variants:

spec/build/bsps/powerpc/motorola_powerpc/boot.yml

@@ -19,7 +19,7 @@ do-build: |
         if f != "-meabi":
             flags += " " + f
   asmflags = "-DASM " + flags
-  cflags = "-msoft-float -mstrict-align -fno-builtin -Wall -mmultiple -mstring -O2 -fomit-frame-pointer -ffixed-r13 -mno-sdata " + flags
+  cflags = "-msoft-float -mstrict-align -fno-builtin -Wall -mmultiple -O2 -fomit-frame-pointer -ffixed-r13 -mno-sdata " + flags
   source = []
   source.append(cc(self, bld, bic, asmflags, "bsps/powerpc/motorola_powerpc/bootloader/head.S"))
   source.append(cc(self, bld, bic, asmflags, "bsps/powerpc/motorola_powerpc/bootloader/exception.S"))