powerpc/signal: Add helper function to fetch quad word aligned pointer
powerpc/signal: Add helper function to fetch quad word aligned pointer

This patch adds one helper function 'sigcontext_vmx_regs' which computes
quad word aligned pointer for 'vmx_reserve' array element in sigcontext
structure making the code more readable.

Signed-off-by: Anshuman Khandual <5cc840ad174e421baefc61e26aff2d5fb1883945@linux.vnet.ibm.com>
[mpe: Reword comment and fix build for CONFIG_ALTIVEC=n]
Signed-off-by: Michael Ellerman <864f124608374e06e4da1fa5d2e47ed839b95411@ellerman.id.au>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs