PowerPC uses a slightly different type of floats for their 128 bit long
doubles, as "double-double", with __LDBL_MANT_DIG__ == 106 rather than
__LDBL_MANT_DIG__ == 113 for IEEE 128 bit floats.
This fixes compiling libcxxabi for PowerPC after
3b70715c13876c51542ebfe2e3f4ee908f6785cb.