powerpc/64: Bump SIGSTKSZ and MINSIGSTKSZ

powerpc/64: Bump SIGSTKSZ and MINSIGSTKSZ

The sad tale of SIGSTKSZ and MINSIGSTKSZ is documented in glibc.git
commit f7c399cff5bd ("PowerPC SIGSTKSZ"), which explains why glibc
does not use the kernel defines for these constants.

Since then in fact there has been a further expansion of the signal
stack frame size on little-endian with linux commit
bcf838b1cd83 ("powerpc: Increase stack redzone for 64-bit userspace to
512 bytes"), which has caused it to exceed even the glibc defines.

See kernel commit ba627f821324 ("powerpc: Allow 4224 bytes of stack
expansion for the signal frame") for more details of the history of the
expansion.

Increase MINSIGSTKSZ to 8192 which is double the current glibc value and
fits the current stack frame with room to grow. SIGSTKSZ is set to 4x
the minimum as convention.

glibc will have to be updated as well.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220307182734.289289-1-npiggin@gmail.com