cleanup PT_GNU_STACK size handling

[Alan Modra]

Please fix.  The empty-aligned failures are likely due to lack of
support for -z stack-size, the assembly failures due to lack of a bare
"nop" instruction.

arc-elf  +FAIL: ld-scripts/empty-aligned
d30v-elf  +FAIL: ld-scripts/empty-aligned
dlx-elf  +FAIL: ld-scripts/empty-aligned
fr30-elf  +FAIL: ld-scripts/empty-aligned
frv-elf  +FAIL: ld-scripts/empty-aligned
i370-linux  +ERROR:  /src/binutils-current/ld/testsuite/ld-elf/stack.s: assembly failed
i370-linux  +ERROR:  /src/binutils-current/ld/testsuite/ld-elf/stack.s: assembly failed
i860-stardent-elf  +FAIL: ld-scripts/empty-aligned
i960-elf  +FAIL: ld-scripts/empty-aligned
ia64-linux  +ERROR:  /src/binutils-current/ld/testsuite/ld-elf/stack.s: assembly failed
ia64-linux  +ERROR:  /src/binutils-current/ld/testsuite/ld-elf/stack.s: assembly failed
iq2000-elf  +FAIL: ld-scripts/empty-aligned
mn10200-elf  +FAIL: ld-scripts/empty-aligned
moxie-elf  +FAIL: ld-scripts/empty-aligned
ms1-elf  +FAIL: ld-scripts/empty-aligned
msp430-elf  +FAIL: ld-scripts/empty-aligned
or32-elf  +FAIL: ld-scripts/empty-aligned
pj-elf  +FAIL: ld-scripts/empty-aligned

-- 
Alan Modra
Australia Development Lab, IBM