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