PowerPC: Fix __fe_mask_env export

This patch does not export __fe_mask_env anymore, only providing a
compatibility symbol. It fixes BZ#14143.
PowerPC: remove wrong truncl implementation for PowerPC64

The truncl assembly implementation (sysdeps/powerpc/powerpc64/fpu/s_truncl.S)
returns wrong results for some inputs where first double is a exact integer
and the precision is determined by second long double.

Checking on implementation comments and history, I am very confident the
assembly implementation was based on a version before commit
5c68d401698a58cf7da150d9cce769fa6679ba5f that fixes BZ#2423 (Errors in
long double (ldbl-128ibm) rounding functions in glibc-2.4).

By just removing the implementation and make the build select
sysdeps/ieee754/ldbl-128ibm/s_truncl.c instead it fixes tgammal
issues regarding wrong result sign.
PowerPC: Fix gettimeofday ifunc selection

The IFUNC selector for gettimeofday runs before _libc_vdso_platform_setup where
__vdso_gettimeofday is set. The selector then sets __gettimeofday (the internal
version used within GLIBC) to use the system call version instead of the vDSO one.
This patch changes the check if vDSO is available to get its value directly
instead of rely on __vdso_gettimeofday.

This patch changes it by getting the vDSO value directly.

It fixes BZ#16431.
abilist-pattern configurability

This patch creates implicit rules to match the abifiles if
abilist-pattern is defined in the architecture Makefile. This allows
machine specific Makefiles to define different abifiles names
(for instance *-le.abilist for powerpc64le).
PowerPC: Change powerpc64le start ABI to 2.17.

Backport of 0ff8246327401ae8779e2697d5c7348611cdbf8a.
PowerPC: powerpc64le abilist for 2.17

This patch is the abifiles for powerpc64le based on GLIBC 2.17.