PowerPC: optimized strcspn for PPC64/POWER7

This patch add a optimized strcspn for POWER7 by using a different
algorithm than default implementation: it constructs a table based on
the 'accept' argument and use this table to check for any occurance
on the input string. The idea is similar as x86_64 uses.
For PowerPC some tunings were added, such as unroll loops and align
stack memory to table to 16 bytes (so VSX clean can ran without
alignment issues).

Backport of 6eaf95cbfa0031ea267682dc2c9c17ed3e3dc167
PowerPC: optimized strpbrk for POWER7

This patch add an optimized strpbrk for POWER7 by using a different
algorithm than default implementation: it constructs a table based on
the 'accept' argument and use this table to check for any occurance on
the input string. The idea is similar as x86_64 uses.
For PowerPC some tunings were added, such as unroll loops and memory
clear using VSX instructions.

Backport of 6f23d0939e9651d8ac3c77a835fb6464b35a1dc4