[PowerPC] Support -mabi=ieeelongdouble and -mabi=ibmlongdouble

gcc PowerPC supports 3 representations of long double:

* -mlong-double-64

  long double has the same representation of double but is mangled as `e`.
  In clang, this is the default on AIX, FreeBSD and Linux musl.

* -mlong-double-128

  2 possible 128-bit floating point representations:

  + -mabi=ibmlongdouble
    IBM extended double format. Mangled as `g`
    In clang, this is the default on Linux glibc.
  + -mabi=ieeelongdouble
    IEEE 754 quadruple-precision format. Mangled as `u9__ieee128` (`U10__float128` before gcc 8.2)
    This is currently unavailable.

This patch adds -mabi=ibmlongdouble and -mabi=ieeelongdouble, and thus
makes the IEEE 754 quadruple-precision long double available for
languages supported by clang.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D64283

llvm-svn: 366044